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Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Payables and Accruals [Abstract]
Employee compensation and benefits	$ 2,675	$ 7,896
External research and development services	1,577	1,255
Other operating expenses	140	447
CVR liability, current	556	395
Professional fees	213	219
Total accrued expenses and other current liabilities	$ 5,161	$ 10,212

Net Loss per Share - Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Details) - Legacy Korro [Member] - shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Antidilutive securities excluded from computation of earnings per share	1,281,768	5,417,353
Outstanding Warrant [Member]
Antidilutive securities excluded from computation of earnings per share	8,049	8,049
Unvested Restricted Stock Units [Member]
Antidilutive securities excluded from computation of earnings per share	0	1,465
Outstanding Stock Options [Member]
Antidilutive securities excluded from computation of earnings per share	1,273,719	552,600
Series Seed Preferred Stock [Member]
Antidilutive securities excluded from computation of earnings per share	0	684,739
Series A Convertible Preferred Stock [Member]
Antidilutive securities excluded from computation of earnings per share	0	2,029,666
Series B-1 Preferred Stock [Member]
Antidilutive securities excluded from computation of earnings per share	0	1,104,178
Series B-2 Preferred Stock [Member]
Antidilutive securities excluded from computation of earnings per share	0	1,036,656

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Financial Statement Affected by Adoption of ASC (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets	$ 25,947	$ 27,150
Operating lease liabilities, current portion	$ 1,235	$ 1,991

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 65
-Paragraph 6
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482833/825-10-65-6

Reference 77: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
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-Paragraph 6
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482833/825-10-65-6

Reference 78: http://www.xbrl.org/2003/role/disclosureRef
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Reference 79: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Reference 80: http://www.xbrl.org/2003/role/disclosureRef
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Reference 81: http://www.xbrl.org/2003/role/disclosureRef
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Reference 82: http://www.xbrl.org/2003/role/disclosureRef
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Reference 83: http://www.xbrl.org/2003/role/disclosureRef
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Reference 84: http://www.xbrl.org/2003/role/disclosureRef
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Reference 85: http://www.xbrl.org/2003/role/disclosureRef
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Reference 86: http://www.xbrl.org/2003/role/disclosureRef
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Reference 87: http://www.xbrl.org/2003/role/disclosureRef
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Reference 88: http://www.xbrl.org/2003/role/disclosureRef
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Reference 89: http://www.xbrl.org/2003/role/disclosureRef
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Reference 90: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 65
-Paragraph 2
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Reference 91: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 65
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481925/310-20-65-2

Reference 92: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480424/946-10-50-3

+ Details

Commitments and Contingencies (Additional Information) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($) ft²	Dec. 31, 2023 USD ($)	May 31, 2023 USD ($)
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets	$ 25,947	$ 27,150
Operating lease liability	35,916
Lease rental payments	$ 68,179
Weighted average remaining lease term	9 years 10 months 24 days
Weighted average remaining lease discount rate	11.20%
Loss contingency accrual	$ 0
OKS Facility [Member]
Lessee, Lease, Description [Line Items]
Lease area of land \| ft²	22,561
Lease expiration date	Oct. 20, 2023
Cummings Park Sublease [Member]
Lessee, Lease, Description [Line Items]
Lease area of land \| ft²	18,148
Lease expiration date	Jul. 31, 2024
60 First Street Lease [Member]
Lessee, Lease, Description [Line Items]
Lease remaining terms	10 years
Lease area of land \| ft²	50,453
Operating lease liability			$ 26,800
Lease rental payments			62,100
Lessee, Operating Lease, Remaining Lease Term	10 years
Tenant improvement allowance	$ 13,100		$ 13,100
Tenant improvement allowance utilized	4,800
Future minimum lease payments receivable	$ 8,300

Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 2024

Earnings Per Share [Abstract]

Computation of Diluted Net Loss per Share

The following common stock equivalents have been excluded from the calculation of diluted net loss per share because their effect would be anti-dilutive:


	Three Months Ended March 31,
	2024		2023
Legacy Korro Series Seed Preferred Stock		—		684,739
Legacy Korro Series A Preferred Stock		—		2,029,666
Legacy Korro Series B-1 Preferred Stock		—		1,104,178
Legacy Korro Series B-2 Preferred Stock		—		1,036,656
Unvested Legacy Korro restricted common stock		—		1,465
Outstanding options to purchase common stock		1,273,719		552,600
Outstanding Legacy Korro warrant		8,049		8,049
Total		1,281,768		5,417,353

Schedule of Basic and Diluted Net Loss per Share

Basic and diluted loss per share is computed by dividing net loss by the weighted-average common shares outstanding. The following table sets forth the computation of the Company's basic and diluted net loss per share (in thousands, except share and per share data):


	Three Months Ended March 31,
	2024			2023
Numerator:
Net loss	$	(19,557	)	$	(19,605	)
Denominator:
Weighted-average number of shares outstanding, basic and diluted		8,019,626			271,522
Net loss per share, basic and diluted	$	(2.44	)	$	(72.20	)

Basis of Presentation and Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2024

Accounting Policies [Abstract]

Basis of Presentation and Summary of Significant Accounting Policies

2. Basis of Presentation and Summary of Significant Accounting Policies

Basis of Presentation and Consolidation

The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“GAAP”). These consolidated financial statements have been prepared on the going

concern basis of accounting, which assumes continuity of operations, realization of assets and satisfaction of liabilities in the ordinary course of business.

The consolidated financial statements include the accounts of Korro Bio, Inc. and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated in consolidation.

Unaudited Interim Condensed Consolidated Financial Information

The accompanying condensed consolidated financial statements as of March 31, 2024 and for the three months ended March 31, 2024 and 2023 are unaudited. The financial data and other information contained in the notes hereto as of March 31, 2024 and for the three months ended March 31, 2024 and 2023 are also unaudited. The condensed consolidated balance sheet data as of December 31, 2023 was derived from the Company's audited consolidated financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2023 (the “2023 Form 10-K”)..

The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements, and in the opinion of management, reflect all adjustments, which include any normal recurring adjustments necessary for the fair presentation of the Company's financial position as of March 31, 2024 and the results of its operations and cash flows for the three months ended March 31, 2024 and 2023. These unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31,2023, and the notes thereto, included in the Company's 2023 Form 10-K.

The results for the three months ended March 31, 2024 are not necessarily indicative of results to be expected for the year ended December 31, 2024, or any other interim periods, or any future year or period.

Summary of Significant Accounting Policies

The significant accounting policies used in preparation of the condensed consolidated financial statements are described in the Company's audited consolidated financial statements as of the year end December 31, 2023, and the notes thereto, which are included in the 2023 Form 10-K. There have been no material changes to the significant accounting policies previously disclosed in the 2023 Form 10-K.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision-maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s chief executive officer, views the Company’s operations and manages its business as a single operating segment. The Company operates only in the United States.

Use of Estimates

The preparation of the Company’s consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to the fair value of the common stock prior to the effective date of the Merger; the fair value of the contingent value rights ("CVR") liability and the incremental borrowing rate for determining lease liabilities and right-of-use assets. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it has concluded to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ materially from those estimates or assumptions.

Recent Accounting Pronouncements—Yet to be Adopted

Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption.

In November 2023, the ASU 2023-07, Segment Reporting (Topic 280), Improvements to Reportable Segment Disclosures, which is intended to provide enhanced segment disclosures. The standard will require disclosures about significant segment expenses

and other segment items and identifying the Chief Operating Decision Maker and how they use the reported segment profitability measures to assess segment performance and allocate resources. These enhanced disclosures are required for all public entities on an interim and annual basis, even if they have only a single reportable segment. The standard is effective for years beginning after December 15, 2023, and interim periods within annual periods beginning after December 15, 2024. Early adoption is permitted. The Company is evaluating this standard to determine if adoption will have a material impact on the Company's consolidated financial statements.

Common Stock (Additional Information) (Details) - $ / shares	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Common stock, shares authorized	200,000,000	200,000,000
Common stock, par value	$ 0.001	$ 0.001
Korros [Member]
Dividends Declared	$ 0
Korros [Member] \| Common Stock [Member]
Common stock, shares authorized	200,000,000
Common stock, par value	$ 0.001

Fair Value Measurements

3 Months Ended

Mar. 31, 2024

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The Company measures the fair value of money market funds based on quoted prices in active markets for identical securities. The carrying amounts reflected in the consolidated balance sheets for cash, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values, due to their short-term nature.

Assets and liabilities measured at fair value on a recurring basis as of March 31, 2024 were as follows (in thousands):


	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Financial assets:
Money market funds, included in cash and cash equivalents	$	135,062	$	135,062	$	—	$	—
MS APA asset	$	1,449	$	—	$	—	$	1,449
Total financial assets	$	136,511	$	135,062	$	—	$	1,449

Financial liabilites:
CVR liability	$	1,449	$	—	$	—	$	1,449
Total financial liabilities	$	1,449	$	—	$	—	$	1,449

Assets and liabilities measured at fair value on a recurring basis as of December 31, 2023 were as follows (in thousands):


	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Financial assets:
Money market funds, included in cash and cash equivalents	$	158,706	$	158,706	$	—	$	—
MS APA asset	$	1,448	$	—	$	—	$	1,448
Total financial assets	$	160,154	$	158,706	$	—	$	1,448

Financial liabilites:
CVR liability	$	1,448	$	—	$	—	$	1,448
Total financial liabilities	$	1,448	$	—	$	—	$	1,448

As noted previously in Note 1, at the effective time of the Transaction, the Company entered into the CVR Agreement providing for the payment of certain contingent cash payments equal to the net amount of proceeds actually received by the Company or its subsidiaries after the end of each fiscal quarter following the first anniversary of the closing of the Transaction related to the disposition of assets related to Frequency’s former multiple sclerosis programs, with the time periods and subject to deductions as provided therein. The Company concluded that the CVR is a derivative liability and is accounted for at fair value, which was $1.4 million as of March 31, 2024 and December 31, 2023, of which $0.6 million and $0.4 million is included in accrued expenses and other current liabilities and the remaining $0.8 million and $1.0 million in other non-current liabilities in the consolidated balance sheet as of March 31, 2024 and December 31, 2023. Concurrently, the Company entered into the MS APA with Progentos, whereby Progentos acquired the rights, title and interest in certain assets related to the Company’s MS program. The MS APA included initial consideration of $0.5 million in proceeds that were settled through net cash at the Merger closing and will be entitled to future milestone payments of up to $17.5 million as well as a $0.7 million payment in the event of Progentos closing of an equity financing at or above a specified amount. The Company concluded that the MS APA is a derivative asset and is accounted for at fair value,

which was $1.4 million as of March 31, 2024 and December 31, 2023, of which $0.6 million and $0.4 million is included in prepaid and other current assets and the remaining $0.8 million and $1.0 million in other long-term assets in the consolidated balance sheet as of March 31,2024 and December 31, 2023.

The fair value of the CVR liability and the MS APA are based on significant unobservable inputs, which represent Level 3 measurements within the fair value hierarchy. In determining the fair value of the CVR liability and the MS APA asset, the Company used the income approach, primarily discounted cash flow models. The discounted cash flow models require the use of significant judgment, estimates and assumptions, including the probability of technical and regulatory success, and discount rates. For the three months ended March 31, 2024 there was no change in the aggregate fair value of the CVR liability and MS APA asset. For the year ended December 31, 2023, the aggregate change in fair value of the CVR liability and MS APA asset was $0.1 million.

There were no changes in valuation techniques, nor were there any transfers among the fair value hierarchy levels during the three months ended March 31, 2024 or during the year ended December 31, 2023.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Common Stock (Tables)

3 Months Ended

Mar. 31, 2024

Equity [Abstract]

Schedule of Number of Shares Reserved for Future Issuance

As of March 31, 2024 and December 31, 2023, the Company had reserved for future issuance the following number of shares of common stock:


	March 31, 2024		December 31, 2023
Exercises of outstanding stock options		1,273,719		1,323,151
Exercise of outstanding warrant		8,049		8,049
Future issuances under 2023 Stock Incentive Plan		700,005		255,694
Future issuances under 2023 ESPP Plan		168,667		88,502
Total reserved for future issuance		2,150,440		1,675,396

Commitments and Contingencies - Schedule of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Operating lease costs	$ 2,150	$ 481
Variable Lease Costs	359	258
Short-term Lease Costs	409	181
Total Lease Costs	$ 2,918	$ 920

Commitments and Contingencies - Schedule of Future Minimum Rental Payments (Details) $ in Thousands	Mar. 31, 2024 USD ($)
Lessee, Lease, Description [Line Items]
2024	$ (5,464)
2025	6,247
2026	7,341
2027	7,557
Thereafter	52,498
Total Future Minimum Leases Payments	68,179
Less: Interest	(32,263)
Present Value of Operating Lease Liabilities	$ 35,916

Common Stock

3 Months Ended

Mar. 31, 2024

Equity [Abstract]

Common Stock

7. Common Stock

At the closing of the Merger, the shares of Legacy Korro common stock were converted into shares of the Company's common stock based on the exchange ratio determined in the Merger Agreement.

As of March 31, 2024, the Company was authorized to issue 200,000,000 shares of common stock, $0.001 per value per share. Prior to the Merger, the holders of Legacy Korro’s common stock were subject to and qualified by the rights, powers and preferences of the holders of the Preferred Stock. Holders of common stock are entitled to one vote per share. In addition, holders of common stock are entitled to receive dividends, if and when declared by the Company’s Board of Directors. As of March 31, 2024, no dividends had been declared.

As of March 31, 2024 and December 31, 2023, the Company had reserved for future issuance the following number of shares of common stock:


	March 31, 2024		December 31, 2023
Exercises of outstanding stock options		1,273,719		1,323,151
Exercise of outstanding warrant		8,049		8,049
Future issuances under 2023 Stock Incentive Plan		700,005		255,694
Future issuances under 2023 ESPP Plan		168,667		88,502
Total reserved for future issuance		2,150,440		1,675,396

- Definition

The entire disclosure for equity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-14

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481062/946-235-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481062/946-235-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481004/946-505-50-6

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480237/815-40-50-6

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480008/505-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(e)(1))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//505/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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-Paragraph 13
-Subparagraph (g)
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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Section 50
-Paragraph 14
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Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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-Topic 505
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-Subparagraph (a)
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Reference 18: http://www.xbrl.org/2003/role/disclosureRef
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

+ Details

Stock-based Compensation (Additional Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, reserved for future issuance	2,150,440		1,675,396
Number of Outstanding Option	1,273,719		1,323,151
Weighted-average grant date fair value of stock options granted	$ 21.43	$ 23.49
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period, Intrinsic Value	$ 0.1	$ 0.0
Total unrecognized stock-based compensation expense relating to unvested stock options	$ 9.6
Unrecognized unvested stock options, weighted-average period	3 years
Restricted Stock Units (RSUs) [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Fair value of restricted common stock	$ 0.0	$ 0.1

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 35
-Paragraph 1D
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480483/718-10-35-1D

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
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-Name Accounting Standards Codification
-Section 35
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480483/718-10-35-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
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-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
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-Section 50
-Paragraph 2
-Subparagraph (a)(2)
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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-Section 50
-Paragraph 2
-Subparagraph (a)(3)
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-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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-Section 50
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-Subparagraph (c)(1)(iv)(04)
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-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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Reference 17: http://www.xbrl.org/2003/role/disclosureRef
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Reference 18: http://www.xbrl.org/2003/role/disclosureRef
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Reference 19: http://www.xbrl.org/2003/role/disclosureRef
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Reference 24: http://www.xbrl.org/2003/role/disclosureRef
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Reference 25: http://www.xbrl.org/2003/role/disclosureRef
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Reference 26: http://www.xbrl.org/2003/role/disclosureRef
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Reference 27: http://www.xbrl.org/2003/role/disclosureRef
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Reference 28: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
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-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(v)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

Stock-based Compensation - Summary of Stock Option Weighted-Average Assumptions (Details)	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-Based Payment Arrangement [Abstract]
Risk-free interest rate	4.10%	3.70%
Expected dividend yield	0.00%	0.00%
Expected term (in years)	6 years	6 years
Expected volatility	74.10%	69.70%

Reverse Merger (Additional Information) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]
Cash and cash equivalents	$ 138,804	$ 166,150

Preferred Stock

3 Months Ended

Mar. 31, 2024

Class of Stock Disclosures [Abstract]

Preferred Stock

8. Preferred Stock

As of March 31, 2024, the Company was authorized to issue up to 10,000,000 shares of preferred stock at a par value of $0.001 with no preferred stock shares issued or outstanding.

Pursuant to the terms of the Merger Agreement, immediately prior to closing of the Merger, each share of Legacy Korro's Preferred Stock issued and outstanding immediately prior to the Closing of the Merger was converted into shares of Legacy Korro’s common stock, and then exchanged in the Merger for shares of the Company's common stock using an exchange ratio of 0.049688. The conversion was approved by greater than 66% of the then-outstanding shares of Preferred Stock, voting as a single class on an as-converted to common stock basis.

Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Operating Activities:
Net loss	$ (19,557)	$ (19,605)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash lease expense	1,203	432
Stock-based compensation expense	865	349
Depreciation expense	726	809
Net amortization of premiums and discounts on investments	0	(68)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(290)	(341)
Accounts payable	(2,585)	549
Accrued expenses and other current liabilities	(5,048)	(1,117)
Operating lease liabilities	2,709	(695)
Other non-current assets	86	(2)
Net cash used in operating activities	(21,891)	(19,689)
Investing Activities:
Proceeds from maturities of investments	0	17,000
Purchases of property and equipment	(5,611)	(1,513)
Net cash provided by (used in) investing activities	(5,611)	15,487
Financing Activities:
Proceeds from exercises of stock options	139	51
Other financing activities, net	0	(150)
Net cash provided by financing activities	139	45,401
Net increase (decrease) in cash, cash equivalents and restricted cash	(27,363)	41,199
Cash, cash equivalents and restricted cash, beginning of period	173,119	41,477
Cash, cash equivalents and restricted cash, end of period	145,756	82,676
Non-cash investing and financing activities:
Property and equipment capitalized under tenant improvement allowance	2,555	0
Purchases of property and equipment in accounts payable and accrued expenses	1,942	1,753
Financing costs in accounts payable and accrued expenses	0	42
Supplemental cash flow information:
Cash paid for income taxes	17	0
Cash paid for operating lease liabilities	828	744
Series B-2 Preferred Stock [Member]
Financing Activities:
Proceeds from Series convertible preferred stock, net of issuance costs	$ 0	$ 45,500

Stock-based Compensation

3 Months Ended

Mar. 31, 2024

Share-Based Payment Arrangement [Abstract]

Stock-based Compensation

9. Stock-based Compensation

The Company grants stock-based awards under its 2023 Stock Option and Incentive Plan (the "2023 Plan"), which was approved by the Company’s stockholders in November 2023 and became effective in November 2023 in connection with completion of the Merger. The Company also has outstanding stock option awards under the "Legacy Korro 2019 Plan", the Frequency 2014 Stock Incentive Plan (the "2014 Plan"), the 2019 Incentive Award Plan (the "Frequency 2019 Plan"), but is no longer granting awards under these plans. The Company also has the option to grant awards under the 2023 Employee Stock Purchase Plan (the "2023 ESPP"), which was approved by the Company’s stockholders in November 2023 and became effective in November 2023 in connection with completion of the Merger.

Stock-based Compensation Expense

Total stock-based compensation expense recognized in the consolidated statements of operations and comprehensive loss for the three months ended March 31, 2024 and 2023 was as follows (in thousands):


	Three Months Ended March 31,
	2024		2023
Research and development	$	278	$	90
General and administrative		587		259
Total stock-based compensation expense	$	865	$	349

Restricted Common Stock Activity

Prior to the adoption of the Legacy Korro 2019 Plan, Legacy Korro issued shares of restricted common stock to its founders as well as to certain employees. The restrictions on the common stock generally lapse over two to four years. In the event that a recipient ceases to provide service to Legacy Korro, Legacy Korro has the right to repurchase any unvested shares of restricted common stock at their original purchase price. As a result of this repurchase right, Legacy Korro recorded the issuance of such restricted common stock as a liability in the consolidated balance sheets. Amounts are reclassified to common stock at par and additional paid-in capital as the restricted common stock vests and restrictions lapse. As of December 31, 2023, all such restricted common stock was fully vested.

The aggregate fair value of Legacy Korro restricted common stock that vested during the three months ended March 31, 2024 and 2023, based upon the fair value of the underlying restricted common stock on the day of vesting, was $0 million and less than $0.1 million, respectively.

Stock Option Activity

The fair value of stock options granted during the three months ended March 31, 2024 and 2023 was calculated on the date of grant using the following weighted-average assumptions:


	Three Months Ended March 31,
	2024			2023
Risk-free interest rate		4.1	%		3.7	%
Expected dividend yield		—	%		—	%
Expected term (in years)		6.0			6.0
Expected volatility		74.1	%		69.7	%

Using the Black-Scholes option pricing model, the weighted-average grant date fair value of stock options granted during the three months ended March 31, 2024 and 2023 was $21.43 and $23.49 per share, respectively.

The following table summarizes changes in stock option activity during the three months ended March 31, 2024 (in thousands, except per share amounts):


	Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2023		1,323,151		$	23.49		8.4	$	36,310
Granted		24,926		$	46.65
Exercised		(5,947	)	$	23.38
Forfeited		(32,174	)	$	18.09
Cancelled		(36,237	)	$	117.15
Outstanding as of March 31, 2024		1,273,719		$	21.42		8.5	$	85,695
Exercisable at March 31, 2024		434,915		$	27.76		7.0	$	25,584

The aggregate intrinsic value of options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the common

stock as of the end of the period. The aggregate intrinsic value of stock options exercised during the three months ended March 31, 2024 and 2023 was $0.1 million and $0 million, respectively.

As of March 31, 2024, there was unrecognized stock-based compensation expense related to unvested stock options of $9.6 million, which the Company expects to recognize over a weighted-average period of approximately 3.0 years.

Restricted Cash - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 138,804	$ 166,150
Restricted cash	6,952	6,969
Cash, cash equivalents and restricted cash	$ 145,756	$ 173,119

Restricted Cash

3 Months Ended

Mar. 31, 2024

Cash and Cash Equivalents [Abstract]

Restricted Cash

4. Restricted Cash

As of both March 31, 2024 and December 31, 2023, the Company maintained current restricted cash of $3.6 million and non-current restricted cash of $3.4 million. All restricted cash amounts are comprised solely of letters of credit required pursuant to the Company’s facility leases.

The following table provides a reconciliation of cash, cash equivalents and restricted cash as of March 31, 2024 and December 31, 2023 that sums to the total of the same amounts shown in the consolidated statements of cash flows (in thousands):


	March 31, 2024		December 31, 2023
Cash and cash equivalents	$	138,804	$	166,150
Restricted cash		6,952		6,969
Cash, cash equivalents and restricted cash	$	145,756	$	173,119

Common Stock - Schedule of Number of Shares Reserved for Future Issuance (Details) - shares	Mar. 31, 2024	Dec. 31, 2023
Preferred stock, shares outstanding	0	0
Future issuance of share	8,022,463	8,016,516
Total reserved for future issuance	2,150,440	1,675,396
Exercises of outstanding Shares	8,022,463	8,016,516
2023 Stock Incentive Plan
Future issuance of share	700,005	255,694
Employee Stock Purchase Plan
Future issuance of share	168,667	88,502
Outstanding Warrant [Member]
Exercises of outstanding Shares	8,049	8,049
Stock Options
Exercises of outstanding Shares	1,273,719	1,323,151

Stock-based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares Options , Beginning balance	1,323,151
Number of shares Options , Granted	24,926
Number of shares Options, Exercised	(5,947)
Number of shares Options, Forfeited	(32,174)
Number of shares Options, Cancelled	(36,237)
Number of shares Options, Ending balance	1,273,719	1,323,151
Exercisable at March 31, 2024	434,915
Weighted average exercise price, Beginning balance	$ 23.49
Weighted average exercise price, Granted	46.65
Weighted average exercise price, Exercised	23.38
Weighted average exercise price, Forfeited	18.09
Weighted- Average Exercise Price Cancelled	117.15
Weighted average exercise price, Ending balance	21.42	$ 23.49
Weighted average exercise price, Options Exercisable at March 31, 2024	$ 27.76
Weighted average remaining contractual term (in years), Outstanding	8 years 6 months	8 years 4 months 24 days
Weighted average remaining contractual term (in years), Options exercisable as of March 31, 2024	7 years
Aggregate intrinsic value, Outstanding as of December 31, 2023	$ 36,310
Aggregate intrinsic value, Outstanding as of March 31, 2024	85,695	$ 36,310
Aggregate intrinsic value, Options exercisable as of Exercisable at March 31, 2024	$ 25,584

Accrued Expenses and Other Current Liabilities (Tables)

3 Months Ended

Mar. 31, 2024

Payables and Accruals [Abstract]

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities as of March 31, 2024 and December 31, 2023 were comprised as follows (in thousands):


	March 31, 2024		December 31, 2023
Employee compensation and benefits	$	2,675	$	7,896
External research and development services		1,577		1,255
Other operating expenses		140		447
CVR liability, current		556		395
Professional fees		213		219
Total accrued expenses and other current liabilities	$	5,161	$	10,212

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2024

Fair Value Disclosures [Abstract]

Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis

Assets and liabilities measured at fair value on a recurring basis as of March 31, 2024 were as follows (in thousands):


	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Financial assets:
Money market funds, included in cash and cash equivalents	$	135,062	$	135,062	$	—	$	—
MS APA asset	$	1,449	$	—	$	—	$	1,449
Total financial assets	$	136,511	$	135,062	$	—	$	1,449

Financial liabilites:
CVR liability	$	1,449	$	—	$	—	$	1,449
Total financial liabilities	$	1,449	$	—	$	—	$	1,449

Assets and liabilities measured at fair value on a recurring basis as of December 31, 2023 were as follows (in thousands):


	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Financial assets:
Money market funds, included in cash and cash equivalents	$	158,706	$	158,706	$	—	$	—
MS APA asset	$	1,448	$	—	$	—	$	1,448
Total financial assets	$	160,154	$	158,706	$	—	$	1,448

Financial liabilites:
CVR liability	$	1,448	$	—	$	—	$	1,448
Total financial liabilities	$	1,448	$	—	$	—	$	1,448

Fair Value Measurements (Additional Information) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Tranfer of assets among fair value hierarchy levels	$ 0	$ 0
Prepaid Expense and Other Assets, Current	3,305	3,015
Other non-current liabilities	$ 893	1,053
MS APA Asset [Member]
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Liabilities
Derivative Liability	$ 1,400	1,400
Other current liabilities	600	400
Initial Consideration Amount	$ 500
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Assets
Derivative Asset	$ 1,400	1,400
Other Assets, Current	800	1,000
Prepaid Expense and Other Assets, Current	600	400
Other non-current liabilities	800	1,000
Maximum [Member] \| MS APA Asset [Member]
Future Milestone Payments	17,500
Minimum [Member] \| MS APA Asset [Member]
Future Milestone Payments	700
Fair Value, Inputs, Level 3 [Member] \| MS APA Asset [Member]
Change in fair value of CVR liability	$ 0	$ 100

Accrued Expenses and Other Current Liabilities

3 Months Ended

Mar. 31, 2024

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

6. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities as of March 31, 2024 and December 31, 2023 were comprised as follows (in thousands):


	March 31, 2024		December 31, 2023
Employee compensation and benefits	$	2,675	$	7,896
External research and development services		1,577		1,255
Other operating expenses		140		447
CVR liability, current		556		395
Professional fees		213		219
Total accrued expenses and other current liabilities	$	5,161	$	10,212

Stock-based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation	$ 865	$ 349
Research and Development Expense [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation	278	90
General and Administrative Expense [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation	$ 587	$ 259

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Condensed Consolidated Statements of Operations and Comprehensive Loss (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Operating expenses:
Research and development	$ 13,572	$ 14,694
General and administrative	7,881	5,392
Total operating expenses	21,453	20,086
Loss from operations	(21,453)	(20,086)
Other income:
Other income, net	1,913	481
Total other income, net	1,913	481
Loss before provision for income taxes	(19,540)	(19,605)
Provision for income taxes	(17)	0
Net loss and comprehensive loss	$ (19,557)	$ (19,605)
Net loss per share, basic	$ (2.44)	$ (72.2)
Net loss per share, diluted	$ (2.44)	$ (72.2)
Weighted-average shares used in computing net loss per share, basic	8,019,626	271,522
Weighted-average shares used in computing net loss per share, diluted	8,019,626	271,522

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2024

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment, net, as of March 31, 2024 and December 31, 2023 was comprised as follows (in thousands):


	Estimated Useful Life (in Years)	March 31, 2024			December 31, 2023
Laboratory equipment	5	$	11,586		$	11,187
Furniture and office equipment	4		481			481
Computer equipment	3		241			241
Leasehold improvements	Shorter of useful life or remaining lease term		3,356			3,356
Construction in progress			13,412			8,242
Total property and equipment, gross			29,076			23,507
Less: accumulated depreciation			(8,567	)		(7,842	)
Total property and equipment, net		$	20,509		$	15,665

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Basis of Presentation and Consolidation	Basis of Presentation and Consolidation The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“GAAP”). These consolidated financial statements have been prepared on the going concern basis of accounting, which assumes continuity of operations, realization of assets and satisfaction of liabilities in the ordinary course of business. The consolidated financial statements include the accounts of Korro Bio, Inc. and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated in consolidation
Unaudited Interim Condensed Consolidated Financial Information	Unaudited Interim Condensed Consolidated Financial Information The accompanying condensed consolidated financial statements as of March 31, 2024 and for the three months ended March 31, 2024 and 2023 are unaudited. The financial data and other information contained in the notes hereto as of March 31, 2024 and for the three months ended March 31, 2024 and 2023 are also unaudited. The condensed consolidated balance sheet data as of December 31, 2023 was derived from the Company's audited consolidated financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2023 (the “2023 Form 10-K”).. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements, and in the opinion of management, reflect all adjustments, which include any normal recurring adjustments necessary for the fair presentation of the Company's financial position as of March 31, 2024 and the results of its operations and cash flows for the three months ended March 31, 2024 and 2023. These unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31,2023, and the notes thereto, included in the Company's 2023 Form 10-K. The results for the three months ended March 31, 2024 are not necessarily indicative of results to be expected for the year ended December 31, 2024, or any other interim periods, or any future year or period.
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The significant accounting policies used in preparation of the condensed consolidated financial statements are described in the Company's audited consolidated financial statements as of the year end December 31, 2023, and the notes thereto, which are included in the 2023 Form 10-K. There have been no material changes to the significant accounting policies previously disclosed in the 2023 Form 10-K.
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision-maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s chief executive officer, views the Company’s operations and manages its business as a single operating segment. The Company operates only in the United States.
Use of Estimates	Use of Estimates The preparation of the Company’s consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to the fair value of the common stock prior to the effective date of the Merger; the fair value of the contingent value rights ("CVR") liability and the incremental borrowing rate for determining lease liabilities and right-of-use assets. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it has concluded to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ materially from those estimates or assumptions.
Recent Accounting Pronouncements - Yet to be Adopted	Recent Accounting Pronouncements—Yet to be Adopted Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption. In November 2023, the ASU 2023-07, Segment Reporting (Topic 280), Improvements to Reportable Segment Disclosures, which is intended to provide enhanced segment disclosures. The standard will require disclosures about significant segment expenses and other segment items and identifying the Chief Operating Decision Maker and how they use the reported segment profitability measures to assess segment performance and allocate resources. These enhanced disclosures are required for all public entities on an interim and annual basis, even if they have only a single reportable segment. The standard is effective for years beginning after December 15, 2023, and interim periods within annual periods beginning after December 15, 2024. Early adoption is permitted. The Company is evaluating this standard to determine if adoption will have a material impact on the Company's consolidated financial statements.

- Definition

Disclosure of accounting policy for segment reporting.

+ References + Details

Preferred Stock (Additional Information) (Details) - $ / shares	Mar. 31, 2024	Dec. 31, 2023	Nov. 03, 2023
Class of Stock [Line Items]
Preferred stock, shares issued	0	0
Preferred Stock, Shares Authorized	10,000,000	10,000,000
Preferred stock, par value	$ 0.001	$ 0.001
Preferred Stock, Shares Outstanding	0	0
Preferred Stock, Convertible, Conversion Ratio	4.9688%
Preferred Stock [Member]
Class of Stock [Line Items]
Percentage of outstanding shares of Preferred Stock	66.00%
Preferred Stock, Convertible, Conversion Ratio	4.9688%
Legacy Korro [Member] \| Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Convertible, Conversion Ratio			4.9688%

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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+ Details

Property and Equipment, Net - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 29,076	$ 23,507
Less: accumulated depreciation	(8,567)	(7,842)
Total property and equipment, net	$ 20,509	$ 15,665
Property, Plant and Equipment, Dispositions	Shorter of useful life or remaining lease term	Shorter of useful life or remaining lease term
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 11,586	$ 11,187
Estimated Useful Life (in Years)	5 years	5 years
Furniture and office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 481	$ 481
Estimated Useful Life (in Years)	4 years	4 years
Computer equipment
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 241	$ 241
Estimated Useful Life (in Years)	3 years	3 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 3,356	$ 3,356
Construction in progress
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 13,412	$ 8,242

Property and Equipment , Net

3 Months Ended

Mar. 31, 2024

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net, as of March 31, 2024 and December 31, 2023 was comprised as follows (in thousands):


	Estimated Useful Life (in Years)	March 31, 2024			December 31, 2023
Laboratory equipment	5	$	11,586		$	11,187
Furniture and office equipment	4		481			481
Computer equipment	3		241			241
Leasehold improvements	Shorter of useful life or remaining lease term		3,356			3,356
Construction in progress			13,412			8,242
Total property and equipment, gross			29,076			23,507
Less: accumulated depreciation			(8,567	)		(7,842	)
Total property and equipment, net		$	20,509		$	15,665

As of March 31, 2024 and December 31, 2023, the Company had construction in progress of $13.4 million and $8.2 million, predominately related to laboratory equipment received but not yet installed and capitalizable costs related to the Company’s future corporate headquarters.

Depreciation expense for the three months ended March 31, 2024 and 2023 was $0.7 million and $0.8 million, respectively.

The Company and Liquidity (Additional Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Nov. 03, 2023
Date of incorporation	Nov. 30, 2014
Common stock, shares issued	8,022,463	8,016,516
Exchange Ratio	4.9688%
Cash and cash equivalents	$ 138,804	$ 166,150
Accumulated deficit	202,562	$ 183,005
MS APA Asset [Member]
Initial Consideration Amount	500
MS APA Asset [Member] \| Minimum [Member]
Future Milestone Payments	700
MS APA Asset [Member] \| Maximum [Member]
Future Milestone Payments	$ 17,500
Common Stock [Member] \| Legacy Korro [Member]
Ownership	91.00%
Common Stock [Member] \| Legacy Korro [Member] \| Accredited Investors [Member]
Common stock, shares issued			42,176,255
Common Stock, Value, Subscriptions			$ 117,300
Sale of Stock, Price Per Share			$ 2.78
Common Stock [Member] \| Frequency [Member]
Common stock, shares issued	8,623,645		7,848,776
Ownership	9.00%
Preferred Stock [Member]
Exchange Ratio	4.9688%
Preferred Stock [Member] \| Legacy Korro [Member]
Exchange Ratio			4.9688%

Net Loss per Share

3 Months Ended

Mar. 31, 2024

Earnings Per Share [Abstract]

Net Loss per Share

13. Net Loss per Share

The following common stock equivalents have been excluded from the calculation of diluted net loss per share because their effect would be anti-dilutive:


	Three Months Ended March 31,
	2024		2023
Legacy Korro Series Seed Preferred Stock		—		684,739
Legacy Korro Series A Preferred Stock		—		2,029,666
Legacy Korro Series B-1 Preferred Stock		—		1,104,178
Legacy Korro Series B-2 Preferred Stock		—		1,036,656
Unvested Legacy Korro restricted common stock		—		1,465
Outstanding options to purchase common stock		1,273,719		552,600
Outstanding Legacy Korro warrant		8,049		8,049
Total		1,281,768		5,417,353


	Three Months Ended March 31,
	2024			2023
Numerator:
Net loss	$	(19,557	)	$	(19,605	)
Denominator:
Weighted-average number of shares outstanding, basic and diluted		8,019,626			271,522
Net loss per share, basic and diluted	$	(2.44	)	$	(72.20	)

Condensed Consolidated Balance Sheets (unaudited) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 138,804	$ 166,150
Restricted cash	3,546	3,563
Prepaid expenses and other current assets	3,305	3,015
Total current assets	145,655	172,728
Property and equipment, net	20,509	15,665
Operating lease right-of-use assets	25,947	27,150
Restricted cash, net of current portion	3,406	3,406
Other non-current assets	2,468	2,714
Total assets	197,985	221,663
Current liabilities:
Accounts payable	4,657	7,280
Accrued expenses and other current liabilities	5,161	10,212
Operating lease liabilities, current portion	1,235	1,991
Total current liabilities	11,053	19,483
Operating lease liabilities, net of current portion	34,681	31,216
Other non-current liabilities	893	1,053
Total liabilities	46,627	51,752
Commitments and contingencies (Note 11)
Stockholders' equity
Preferred stock, $0.001 par value; 10,000,000 shares authorized at March 31, 2024 and December 31, 2023; no shares issued and outstanding at March 31, 2024 and December 31, 2023	0	0
Common stock, $0.001 par value; 200,000,000 shares authorized at March 31, 2024 and December 31, 2023; 8,022,463 shares and 8,016,516 shares issued and outstanding at March 31, 2024 and December 31, 2023, respectively	8	8
Additional paid-in capital	353,912	352,908
Accumulated deficit	(202,562)	(183,005)
Total stockholders' equity	151,358	169,911
Total liabilities and stockholders' equity	$ 197,985	$ 221,663

Subsquent Events

3 Months Ended

Mar. 31, 2024

Subsequent Events [Abstract]

Subsquent Events

14. Subsequent Events

On April 17, 2024, the Company entered into a subscription agreement with certain new and existing accredited investors to issue and sell an aggregate of 1,249,283 shares of its common stock in a private placement ("PIPE") that resulted in gross proceeds of approximately $70.0 million, before deducting placement agent fees and estimated offering expenses. The PIPE closed on April 22, 2024.

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Series Seed Convertible Preferred Stock [Member]	Series A Convertible Preferred Stock [Member]	Series B-1 Convertible Preferred Stock [Member]	Series B-2 Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Temporary equity, beginning balance (in share) at Dec. 31, 2022		684,739	2,029,666	1,104,178	223,417
Temporary equity, beginning balance at Dec. 31, 2022		$ 15,924	$ 77,736	$ 57,703	$ 12,500
Beginning balance (in share) at Dec. 31, 2022						268,399
Beginning balance at Dec. 31, 2022	$ (99,031)						$ 2,807	$ (5)	$ (101,833)
Issuance of common stock for services rendered (in share)						292
Issuance of common stock for services rendered	6						6
Exercises of stock options (in shares)						4,332
Exercises of stock options	51						51
Vesting of restricted common stock (in share)						1,437
Stock-based compensation expense	343						343
Issuance of Series B-2 convertible preferred stock, net of issuance costs (in share)					813,239
Issuance of Series B-2 convertible preferred stock, net of issuance costs					$ 45,458
Other comprehensive income	5							5
Net loss	(19,605)								(19,605)
Ending balance (in share) at Mar. 31, 2023						274,460
Ending balance at Mar. 31, 2023	$ (118,231)						3,207	0	(121,438)
Temporary equity, ending balance (in share) at Mar. 31, 2023		684,739	2,029,666	1,104,178	1,036,656
Temporary equity, ending balance at Mar. 31, 2023		$ 15,924	$ 77,736	$ 57,703	$ 57,958
Temporary equity, beginning balance (in share) at Dec. 31, 2023		0	0	0	0
Temporary equity, beginning balance at Dec. 31, 2023		$ 0	$ 0	$ 0	$ 0
Beginning balance (in share) at Dec. 31, 2023	8,016,516					8,016,516
Beginning balance at Dec. 31, 2023	$ 169,911					$ 8	352,908	0	(183,005)
Exercises of stock options (in shares)	5,947					5,947
Exercises of stock options	$ 139						139
Stock-based compensation expense	865						865
Net loss	$ (19,557)								(19,557)
Ending balance (in share) at Mar. 31, 2024	8,022,463					8,022,463
Ending balance at Mar. 31, 2024	$ 151,358					$ 8	$ 353,912	$ 0	$ (202,562)
Temporary equity, ending balance (in share) at Mar. 31, 2024		0	0	0	0
Temporary equity, ending balance at Mar. 31, 2024		$ 0	$ 0	$ 0	$ 0

Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2024	May 09, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Amendment Flag	false
Document Period End Date	Mar. 31, 2024
Document Fiscal Year Focus	2024
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q1
Entity File Number	001-39062
Entity Registrant Name	Korro Bio, Inc.
Entity Central Index Key	0001703647
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-2324450
Entity Address, Address Line One	One Kendall Square
Entity Address, Address Line Two	Building 600-700
Entity Address, Address Line Three	Suite 6-401
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	468-1999
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	KRRO
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Document Transition Report	false
Entity Small Business	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		9,272,987

Genevant Agreement (Additional Information) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense, Total	$ 13,572	$ 14,694
Reimbursements with in Research and Development Expense	900	$ 2,500
Genevant Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense, Total			$ 2,500
Revenue recognized commercialization milestone payments to receive	57,000
Genevant Agreement [Member] \| Maximum [Member] \| Phase 3 Clinical Trial
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	13,500
Genevant Agreement [Member] \| Maximum [Member] \| Phase2b Clinical Trial
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 27,000

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2024

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Rental Payments

Future minimum lease payments for all three leases, net of $8.3 million expected to be received and intended to be used related to the remaining tenant improvement allowance and rent credits associated with the 60 First Street Lease, as of March 31, 2024 were as follows (in thousands):


	As of March 31, 2024
2024		(5,464	)
2025		6,247
2026		7,341
2027		7,557
Thereafter		52,498
Total Future Minimum Leases Payments		68,179
Less: Interest		(32,263	)
Present Value of Operating Lease Liabilities		35,916

Schedule of Lease Costs

The following table summarizes the effect of lease costs in the Company’s consolidated statement of operations and comprehensive loss of its operating leases (in thousands):


	Three Months Ended March 31,
	2024		2023
Operating lease costs	$	2,150	$	481
Variable Lease Costs		359		258
Short-term Lease Costs		409		181
Total Lease Costs	$	2,918	$	920

401(k) Savings Plan (Additional Information) (Details) - USD ($) $ in Millions		3 Months Ended
	Apr. 01, 2022	Mar. 31, 2024	Mar. 31, 2023
Defined Contribution Plan [Table]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	3.00%
Defined Contribution Plan, Cost		$ 0.2	$ 0.2
Maximum [Member]
Defined Contribution Plan [Table]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100.00%

Income Taxes - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Deferred taxes:
Total provision for income taxes	$ 17	$ 0

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References + Details

401(k) Savings Plan

3 Months Ended

Mar. 31, 2024

Retirement Benefits [Abstract]

401(k) Savings Plan

12. 401(k) Savings Plan

The Company has a defined-contribution savings plan under Section 401(k) of the IRC (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation, subject to statutory limitations. Beginning on April 1, 2022, the Company matches 100% of an employee’s 401(k) contributions up to a maximum of 3% of the participant’s salary, subject to employer match limitations under the IRC. As such, the Company made $0.2 million and $0.2 million of matching contributions to the 401(k) Plan during the three months ended March 31, 2024 and March 31, 2023, respectively.

Restricted Cash (Tables)

3 Months Ended

Mar. 31, 2024

Cash and Cash Equivalents [Abstract]

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash


	March 31, 2024		December 31, 2023
Cash and cash equivalents	$	138,804	$	166,150
Restricted cash		6,952		6,969
Cash, cash equivalents and restricted cash	$	145,756	$	173,119

Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value Measurements Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	$ 136,511	$ 160,154
Total financial liabilities	1,449	1,448
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	135,062	158,706
Total financial liabilities	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	0	0
Total financial liabilities	0	0
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	1,449	1,448
Total financial liabilities	1,449	1,448
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	135,062	158,706
Money Market Funds [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	135,062	158,706
Money Market Funds [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	0	0
Money Market Funds [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	0	0
MS APA Asset [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	1,449	1,448
MS APA Asset [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	0	0
MS APA Asset [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	0	0
MS APA Asset [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	1,449	1,448
CVR liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial liabilities	1,449	1,448
CVR liability \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial liabilities	0	0
CVR liability \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial liabilities	0	0
CVR liability \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial liabilities	$ 1,449	$ 1,448

Net Loss per Share - Schedule of Basic And Diluted Net Loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Numerator:
Net loss	$ (19,557)	$ (19,605)
Denominator:
Weighted-average shares used in computing net loss per share, basic	8,019,626	271,522
Weighted-average shares used in computing net loss per share, diluted	8,019,626	271,522
Net loss per share, basic	$ (2.44)	$ (72.2)
Net loss per share, diluted	$ (2.44)	$ (72.2)

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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+ Details

Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2024

Share-Based Payment Arrangement [Abstract]

Summary of Stock-based Compensation Expense

Total stock-based compensation expense recognized in the consolidated statements of operations and comprehensive loss for the three months ended March 31, 2024 and 2023 was as follows (in thousands):


	Three Months Ended March 31,
	2024		2023
Research and development	$	278	$	90
General and administrative		587		259
Total stock-based compensation expense	$	865	$	349

Summary of Stock Option Weighted-Average Assumptions

The fair value of stock options granted during the three months ended March 31, 2024 and 2023 was calculated on the date of grant using the following weighted-average assumptions:


	Three Months Ended March 31,
	2024			2023
Risk-free interest rate		4.1	%		3.7	%
Expected dividend yield		—	%		—	%
Expected term (in years)		6.0			6.0
Expected volatility		74.1	%		69.7	%

Summary of Stock Option Activity

The following table summarizes changes in stock option activity during the three months ended March 31, 2024 (in thousands, except per share amounts):


	Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2023		1,323,151		$	23.49		8.4	$	36,310
Granted		24,926		$	46.65
Exercised		(5,947	)	$	23.38
Forfeited		(32,174	)	$	18.09
Cancelled		(36,237	)	$	117.15
Outstanding as of March 31, 2024		1,273,719		$	21.42		8.5	$	85,695
Exercisable at March 31, 2024		434,915		$	27.76		7.0	$	25,584

Subsequent Events (Additional Information) (Details) - USD ($) $ in Millions	Apr. 17, 2024	Mar. 31, 2024	Dec. 31, 2023
Subsequent Event [Line Items]
Common Stock, Shares, Issued		8,022,463	8,016,516
Subscription agreement [Member] \| Private Placement [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Proceeds from Issuance of Private Placement	$ 70.0
Common Stock, Shares, Issued	1,249,283

Commitments and Contingencies

3 Months Ended

Mar. 31, 2024

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Leases

The Company is party to an operating lease at One Kendall Square, Cambridge, Massachusetts and occupies 22,561 square feet of laboratory and office space (the “OKS Facility”). On October 20, 2023, the Company entered into an amendment to the Lease Agreement that extended the lease expiration date from December 31, 2023 to September 30, 2024 and provided the Company with no option to further extend the lease expiration date. The Company is also party to an operating sublease agreement at Cummings Park in Woburn, Massachusetts and occupies 18,148 square feet of laboratory and office space (the “Cummings Park Sublease”), which expires on July 31, 2024.

The Company is party to an operating lease for 50,453 square feet of office and laboratory space at 60 First Street, Cambridge, Massachusetts (the “60 First Street Lease”). In May 2023, the Company obtained control over the space and the Company recognized the operating lease right-of-use asset and the operating lease liability of $26.8 million on the commencement date of the lease. The total rental payments over the 11 year lease are expected to be $62.1 million, including rent credits and other lease incentives per the terms of the lease. Specifically, the 60 First Street Lease provides the Company with a tenant improvement allowance of $13.1 million. The Company utilized $4.8 million of the $13.1 million tenant improvement allowance as of March 31, 2024. The lease has remaining term of approximately 10 years. The Company has an option to extend the lease for an additional period of five years with the rent during the option period being the then fair market rent.


	As of March 31, 2024
2024		(5,464	)
2025		6,247
2026		7,341
2027		7,557
Thereafter		52,498
Total Future Minimum Leases Payments		68,179
Less: Interest		(32,263	)
Present Value of Operating Lease Liabilities		35,916

As of March 31, 2024, the weighted average remaining lease term was 9.9 years and the weighted average incremental borrowing rate used to determine the operating lease liability was 11.2%.

The Company combines the lease and non-lease components of fixed costs in its lease arrangements as a single lease component. Variable costs, such as utilities and maintenance costs, are not included in the measurement of right-of-use assets and lease liabilities, but rather are expensed when the event determining the amount of variable consideration to be paid occurs.

The following table summarizes the effect of lease costs in the Company’s consolidated statement of operations and comprehensive loss of its operating leases (in thousands):


	Three Months Ended March 31,
	2024		2023
Operating lease costs	$	2,150	$	481
Variable Lease Costs		359		258
Short-term Lease Costs		409		181
Total Lease Costs	$	2,918	$	920

Legal Contingencies

The Company accrues a liability for legal contingencies when it believes that it is both probable that a liability has been incurred and that the Company can reasonably estimate the amount of the loss. The Company reviews these accruals and adjusts them to reflect ongoing negotiations, settlements, rulings, advice of legal counsel and other relevant information. To the extent new information is obtained and the views on the probable outcomes of claims, suits, assessments, investigations or legal proceedings change, changes in the Company’s accrued liabilities would be recorded in the period in which such determination is made.

In addition, in accordance with the relevant authoritative guidance, for any matters in which the likelihood of material loss is at least reasonably possible, the Company will provide disclosure of the possible loss or range of loss. If a reasonable estimate cannot be made, however, the Company will provide disclosure to that effect. The Company expenses legal costs as they are incurred.

On June 3, 2021 and June 22, 2021, purported stockholders of the Company filed putative class action lawsuits in the U.S. District Court for the District of Massachusetts against the Frequency and the Frequency’s Chief Executive Officer, President, and Director, David Lucchino. On March 21, 2022, the two lawsuits were consolidated into a single lawsuit, Quinones et al. v. Frequency Therapeutics, Inc. et al. and on May 16, 2022, the Company’s Chief Development Officer, Dr. Carl Le Bel, was added as a defendant. The plaintiffs alleged violations of Sections 10(b), 20(a) and Rule 10b5 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), due to allegedly false and misleading statements and omissions about the Company’s Phase 2a clinical trial (FX-322-202) for its product candidate FX-322 in the Company’s public disclosures between October 29, 2020 and March 22,2021. The lawsuit sought, among other things, damages in connection with the Company’s allegedly artificially inflated stock price between October 29,2020 and March 22, 2021 as a result of those allegedly false and misleading statements and omissions, as well as interest, attorneys’ fees and costs. The Company filed a motion to dismiss the Amended Complaint on July 15, 2022. On March 29, 2023, the Company’s motion to dismiss was granted and the lawsuit was dismissed in its entirety. On April 27, 2023, Plaintiff filed a notice of appeal to the United States Court of Appeals for the First Circuit from the order dismissing the lawsuit. On August 2, 2023, Plaintiff-Appellant submitted its opening brief to the First Circuit. The Company filed its opposition brief on October 27, 2023, and Plaintiff-Appellant filed its reply brief on December 14, 2023. The First Circuit heard oral argument on January 8, 2024, and has not yet issued

a decision on plaintiff’s appeal. This matter is at the very early stages of the legal process, and as a result, the Company is not able to estimate a range of possible loss. Because an estimate of the possible loss or range of loss cannot be made at this time, no accruals have been recorded as of March 31, 2024.

Indemnifications

The Company indemnifies each of its officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with the Company’s amended and restated certificate of incorporation and bylaws. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity.

The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with the Company’s exposure and may enable the Company to recover a portion of any future amounts paid. The Company believes that the fair value of these potential indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

Restricted Cash (Additional Information) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Cash and Cash Equivalents [Abstract]
Restricted Cash, Current	$ 3,546	$ 3,563
Restricted Cash, Noncurrent		$ 3,400

Condensed Consolidated Balance Sheets (unaudited) (Parenthetical) - $ / shares	Mar. 31, 2024	Dec. 31, 2023
Statement [Table]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	8,022,463	8,016,516
Common stock, shares outstanding	8,022,463	8,016,516

- Definition

Schedule reflecting a Statement of Income, Statement of Cash Flows, Statement of Financial Position, Statement of Shareholders' Equity and Other Comprehensive Income, or other statement as needed.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

The Company and Liquidity

3 Months Ended

Mar. 31, 2024

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The Company and Liquidity

1. The Company and Liquidity

Nature of Business

Korro Bio, Inc. (together with its subsidiaries, the “Company”) is a biopharmaceutical company with a mission to discover, develop and commercialize a new class of genetic medicines based on editing RNA, enabling the treatment of both rare and highly prevalent diseases. The Company was incorporated in November 2014 as a Delaware corporation. The Company's principal offices are in Cambridge, Massachusetts.

Risks and Uncertainties

The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive pre-clinical and clinical testing and regulatory approval, prior to commercialization. These efforts will require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from product sales.

Reverse Merger with Legacy Korro

On July 14, 2023, Frequency Merger Sub, Inc.(“Merger Sub”) a Delaware corporation, a wholly-owned subsidiary of Frequency Therapeutics, Inc. (“Frequency”), a Delaware corporation and Korro Bio Ops, Inc (“Legacy Korro”), a Delaware corporation, entered into an Agreement and Plan of Merger (the “Merger Agreement”). The merger was completed on November 3, 2023 (the “Merger” or the "Transaction"). In accordance with the Merger Agreement, the Merger Sub merged with and into Legacy Korro, with Legacy Korro surviving as a wholly-owned subsidiary of the Company. In connection with the completion of the Merger, the Company changed its name from Frequency Therapeutics, Inc. to Korro Bio, Inc. On November 6, 2023, the combined company’s common stock began trading on The Nasdaq Capital Market under the ticker symbol “KRRO”.

Except as otherwise indicated, references herein to the “Company,” or the “combined company”, refer to Korro Bio, Inc. on a post-merger basis, and the term “Legacy Korro” refers to the business of privately held Korro Bio Ops, Inc., (formerly known as Korro Bio, Inc.), prior to completion of the Merger. References to Frequency refer to Frequency Therapeutics, Inc. prior to completion of the Merger.

Concurrently with the execution and delivery of the Merger Agreement, Legacy Korro entered into a subscription agreement with a number of accredited investors. Immediately prior to consummation of the Merger, Legacy Korro issued and sold an aggregate of 42,176,255 shares of its common stock at a purchase price of approximately $2.78 per share, for an aggregate purchase price of approximately $117.3 million (the “Pre-Closing Financing”). Shares of Legacy Korro common stock issued pursuant to the Pre-Closing Financing were converted into shares of the Company’s common stock based on an exchange ratio (as defined below).

Pursuant to the terms of the Merger Agreement, immediately prior to the effective time of the Merger, each share of Legacy Korro preferred stock was converted into a share of Legacy Korro common stock, and then exchanged in the Merger for shares of Frequency common stock using an exchange ratio of 0.049688 (the “Exchange Ratio”).

At the effective time of the Merger, the Company issued (or reserved for issuance upon exercise of options assumed in the Merger) an aggregate of approximately 7,848,776 shares of its common stock to Legacy Korro securityholders (before eliminating fractions), calculated as provided in the Merger Agreement, (the “Exchange”), resulting in approximately 8,623,645 shares of its common stock being issued and outstanding on a fully diluted basis immediately following the effective time of the Merger. This number includes shares of the Company’s common stock that was issued upon vesting and settlement of certain outstanding equity awards at the effective time of the Merger. Immediately following the completion of the Merger, Frequency securityholders prior to the Merger owned approximately 9% of the outstanding shares of common stock on a fully diluted basis and Legacy Korro’s securityholders, including those securityholders who purchased shares in the Pre-Closing Financing, owned approximately 91% of the outstanding shares on a fully diluted basis. The Merger was intended to qualify for federal income tax purposes as a tax-free reorganization under the provisions of Section 368(a) of the Internal Revenue Code of 1986, as amended (the "IRC").

Upon closing of the Merger, the Company assumed the Legacy Korro 2019 Stock Incentive Plan (the “Legacy Korro 2019 Plan”), and each outstanding and unexercised option to purchase Legacy Korro shares at such time, each of which converted into an

option to purchase shares of the Company’s common stock, with necessary adjustments to the number of shares and exercise price to reflect the Exchange Ratio. In addition, upon the closing of the Merger, the Company assumed each outstanding and unexercised warrant to purchase Legacy Korro shares at such time, each of which converted into a warrant to purchase shares of the Company’s common stock, with necessary adjustments to the number of shares and exercise price to reflect the Exchange Ratio.

At the effective time of the Merger, the Company entered into a contingent value rights agreement (the “CVR Agreement”) with Computershare Trust Company, N.A. and Computershare Inc., collectively as rights agent providing for the payment of certain contingent cash payments equal to the net amount (calculated in accordance with GAAP consistently applied) of proceeds actually received by the Company or its subsidiaries after the end of each fiscal quarter following the first anniversary of the closing of the Transaction related to the disposition of assets related to Frequency’s former multiple sclerosis programs, with the time periods and subject to deductions as provided therein. Concurrently, the Company entered into an asset purchase agreement with Progentos Therapeutics (“Progentos”), whereby Progentos acquired the rights, title and interest in certain assets related to Freqeuncy’s MS program ("MS APA"). The MS APA included initial consideration of $0.5 million in proceeds that were settled through net cash at the Merger closing and will be entitled to future milestone payments of up to $17.5 million as well as a $0.7 million payment in the event of Progentos closing of an equity financing at or above a specified amount.

The Merger was accounted for as a reverse recapitalization in accordance with U.S. GAAP. For accounting purposes, Legacy Korro was considered to be acquiring the assets and liabilities of Frequency in the Merger based on the terms of the Merger Agreement and other factors, including: (i) Legacy Korro controlling the majority of outstanding voting shares; (ii) Legacy Korro controlling the Board of Directors; (iii) Legacy Korro’s executive management team became the management of the combined company; and (iv) the pre-combination assets of Frequency were primarily cash and cash equivalents and other non-operating assets. Accordingly, the Merger was treated as the equivalent of Legacy Korro issuing stock to acquire the net assets of Frequency. As a result of Legacy Korro being treated as the accounting acquirer, Legacy Korro’s assets and liabilities were recorded at their pre-combination carrying amounts. Frequency’s assets and liabilities were measured and recognized at their fair values, which approximated their carrying values as of the effective date of the Merger, and combined with the assets, liabilities, and results of operations of Legacy Korro after the consummation of the Merger. As a result, upon consummation the historical financial statements of Legacy Korro became the historical consolidated financial statements of the combined company.

Reverse Stock Split and Exchange Ratio

In connection with, and prior to the completion of, the Merger, Frequency effected a 1-for-50 reverse stock split of its then outstanding common stock (the “Reverse Stock Split”). The par value and the authorized shares of the common stock were not adjusted as a result of the Reverse Stock Split. All issued and outstanding Legacy Korro common stock, convertible preferred stock and options prior to the effective date of the Merger have been retroactively adjusted to reflect the Merger 0.049688 Exchange Ratio, which reflects the impact of the reverse stock split, for all periods presented.

Liquidity and Capital Resources

The Company’s consolidated financial statements have been prepared on the basis of the Company continuing as a going concern. The Company expects that its existing cash and cash equivalents as of March 31, 2024 of $138.8 million will enable the Company to fund its planned operating expense and capital expenditure requirements for at least twelve months from the date of issuance of these consolidated financial statements. The Company has incurred recurring losses and negative cash flows from operations since inception. As of March 31, 2024, the Company had an accumulated deficit of $202.6 million. The Company expects its operating losses and negative operating cash flows to continue into the foreseeable future. The future viability of the Company is dependent on its ability to generate cash from operating activities or to raise additional capital to finance its operations. There can be no assurance that the Company will ever earn revenues or achieve profitability, or if achieved, that the revenues or profitability will be sustained on a continuing basis. In addition, the Company’s preclinical and clinical development activities, manufacturing and commercialization of the Company’s product candidates, if approved, will require significant additional financing. However, if the Company is unable to obtain additional financing, the Company would be forced to delay, reduce or eliminate its research and development programs and/or relinquish valuable rights to its technology and product candidates. There is no assurance that the Company will be successful in obtaining sufficient financing on acceptable terms to continue funding its operations.

Genevant Agreement

3 Months Ended

Mar. 31, 2024

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Genevant Agreement

10. Genevant Agreement

In March 2023, Legacy Korro entered into a collaboration and license agreement (the “Genevant Agreement”) with Genevant Sciences GmbH (“Genevant”). Key financial terms under the Genevant Agreement are as follows:

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The Company made a $2.5 million payment to Genevant in March 2023 upon execution of the Genevant Agreement and recorded the payment within research and development expense in the consolidated statement of operations for the year ended December 31, 2023.

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The Company will reimburse Genevant for certain out-of-pocket and full-time equivalent costs incurred as a result of research and development activities performed under the Genevant Agreement.

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Genevant is entitled to receive payments from the Company upon the achievement of certain milestones, including potential clinical milestone payments of up to $13.5 million, potential regulatory and development milestone payments of up to $27.0 million, and potential commercial milestone payments up to an aggregate total of $57.0 million.

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Genevant is eligible to receive royalties at percentage rates in the mid-single-digits, based on future annual net sales of licensed products within the scope of the Genevant Agreement.

As of March 31, 2024, no milestones have been achieved and the Company has recorded payments to Genevant of $0.9 million and $2.5 million within research and development expense in the consolidated statement of operations during the three months ended March 31, 2024 and 2023, respectively.

Property and Equipment, Net (Additional Information) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 726	$ 809
Construction in progress	$ 13,400		$ 8,200