Korro Bio (KRRO) S-1 14 May 24 IPO registration Financial data

Cover Page

Cover Page	3 Months Ended
	Mar. 31, 2024
Document Information [Line Items]
Document Type	S-1
Amendment Flag	false
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
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Entity Small Business	true
Entity Ex Transition Period	false
Entity Primary SIC Number	2834
Business Contact [Member]
Document Information [Line Items]
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
Contact Personnel Name	Ram Aiyar

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 138,804	$ 166,150	$ 36,333
Short-term investments		0	18,915
Restricted cash	3,546	3,563	603
Prepaid expenses and other current assets	3,305	3,015	1,232
Total current assets	145,655	172,728	57,083
Property and equipment, net	20,509	15,665	9,866
Operating lease right-of-use assets	25,947	27,150	2,024
Restricted cash, net of current portion	3,406	3,406	4,541
Other non-current assets	2,468	2,714	228
Total assets	197,985	221,663	73,742
Current liabilities:
Accounts payable	4,657	7,280	2,605
Accrued expenses and other current liabilities	5,161	10,212	3,175
Operating lease liabilities, current portion	1,235	1,991	2,921
Total current liabilities	11,053	19,483	8,701
Operating lease liabilities, net of current portion	34,681	31,216	209
Other non-current liabilities	893	1,053	0
Total liabilities	46,627	51,752	8,910
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	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	0
Additional paid-in capital	353,912	352,908	2,807
Accumulated other comprehensive loss		0	(5)
Accumulated deficit	(202,562)	(183,005)	(101,833)
Total stockholders' equity	151,358	169,911	(99,031)
Total liabilities and stockholders' equity	$ 197,985	221,663	73,742
Series Seed Convertible Preferred Stock [Member]
Current liabilities:
Temporary equity, convertible preferred stock		0	15,924
Series A Preferred Stock [Member]
Current liabilities:
Temporary equity, convertible preferred stock		0	77,736
Series B-1 Preferred Stock [Member]
Current liabilities:
Temporary equity, convertible preferred stock		0	57,703
Series B-2 Preferred Stock [Member]
Current liabilities:
Temporary equity, convertible preferred stock		$ 0	$ 12,500

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	0
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	5,755,759
Common stock, shares issued	8,016,516	271,371
Common stock, shares outstanding	8,016,516	268,468
Series Seed Convertible Preferred Stock [Member]
Temporary equity, preferred stock, par value	$ 0.001	$ 0.001
Temporary equity, preferred stock, shares authorized	0	684,739
Temporary equity, preferred stock, shares issued	0	684,739
Temporary equity, preferred stock, shares outstanding	0	684,739
Temporary equity, preferred stock, liquidation preference	$ 0	$ 16,115
Series A Convertible Preferred Stock [Member]
Temporary equity, preferred stock, par value	$ 0.001	$ 0.001
Temporary equity, preferred stock, shares authorized	0	2,029,666
Temporary equity, preferred stock, shares issued	0	2,029,666
Temporary equity, preferred stock, shares outstanding	0	2,029,666
Temporary equity, preferred stock, liquidation preference	$ 0	$ 91,500
Preferred stock, shares outstanding	0	2,029,666
Series B-1 Preferred Stock [Member]
Temporary equity, preferred stock, par value	$ 0.001	$ 0.001
Temporary equity, preferred stock, shares authorized	0	1,104,178
Temporary equity, preferred stock, shares issued	0	1,104,178
Temporary equity, preferred stock, shares outstanding	0	1,104,178
Temporary equity, preferred stock, liquidation preference	$ 0	$ 58,000
Series B-2 Preferred Stock [Member]
Temporary equity, preferred stock, par value	$ 0.001	$ 0.001
Temporary equity, preferred stock, shares authorized	0	1,036,656
Temporary equity, preferred stock, shares issued	0	223,417
Temporary equity, preferred stock, shares outstanding	0	223,417
Temporary equity, preferred stock, liquidation preference	$ 0	$ 12,500

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Operating expenses:
Research and development	$ 13,572	$ 14,694	$ 57,250	$ 42,201
General and administrative	7,881	5,392	27,284	16,797
Total operating expenses	21,453	20,086	84,534	58,998
Loss from operations	(21,453)	(20,086)	(84,534)	(58,998)
Other income:
Other income, net	1,913	481	3,389	976
Total other income, net	1,913	481	3,389	976
Loss before provision for income taxes	(19,540)	(19,605)	(81,145)	(58,022)
Provision for income taxes	(17)	0	27	10
Net loss and comprehensive loss	$ (19,557)	$ (19,605)	(81,172)	(58,032)
Other comprehensive income
Unrealized gain on available-for-sale investments			0	2
Comprehensive loss			$ (81,172)	$ (58,030)
Net loss per share, basic	$ (2.44)	$ (72.2)	$ (53.08)	$ (227.42)
Net loss per share, diluted	$ (2.44)	$ (72.2)	$ (53.08)	$ (227.42)
Weighted-average shares used in computing net loss per share, basic	8,019,626	271,522	1,529,321	255,175
Weighted-average shares used in computing net loss per share, diluted	8,019,626	271,522	1,529,321	255,175

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity - 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, 2021		684,739	2,029,666	1,104,178	223,417
Temporary equity, beginning balance at Dec. 31, 2021		$ 15,924	$ 77,736	$ 57,703	$ 12,500
Beginning balance (in share) at Dec. 31, 2021						237,803
Beginning balance at Dec. 31, 2021	$ (42,208)						$ 1,600	$ (7)	$ (43,801)
Issuance of common stock for services rendered (in share)						497
Issuance of common stock for services rendered	11						11
Exercises of stock options (in shares)						8,722
Exercises of stock options	62						62
Vesting of restricted common stock (in share)						21,377
Vesting of restricted common stock	5						5
Stock-based compensation expense	1,129						1,129
Other comprehensive income	2							2
Net loss	$ (58,032)								(58,032)
Ending balance (in share) at Dec. 31, 2022	268,468					268,399
Ending balance at Dec. 31, 2022	$ (99,031)					$ 0	2,807	(5)	(101,833)
Temporary equity, ending balance (in share) at Dec. 31, 2022		684,739	2,029,666	1,104,178	223,417
Temporary equity, ending balance at Dec. 31, 2022		$ 15,924	$ 77,736	$ 57,703	$ 12,500
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)					$ 0	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, 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	268,468					268,399
Beginning balance at Dec. 31, 2022	$ (99,031)					$ 0	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)	39,395					73,394
Exercises of stock options	$ 702						702
Vesting of restricted common stock (in share)						2,902
Stock-based compensation expense	2,375						2,375
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
Issuance of common stock for cash in PIPE financing (in share)						2,077,864
Issuance of common stock for cash in PIPE financing	117,252					$ 2	117,250
Conversion of preferred stock to common stock (in share)		(684,739)	(2,029,666)	(1,104,178)	(1,036,656)	4,855,208
Conversion of preferred stock to common stock	209,320	$ (15,924)	$ (77,736)	$ (57,703)	$ (57,958)	$ 5	209,315
Issuance of common stock to Frequency shareholders in reverse recapitalization (in share)						738,457
Issuance of common stock to Frequency shareholders in reverse recapitalization	32,701					$ 1	32,700
Reverse recapitalization transaction costs	(12,247)						(12,247)
Other comprehensive income	5							5
Net loss	$ (81,172)								(81,172)
Ending balance (in share) at Dec. 31, 2023	8,016,516					8,016,516
Ending balance at Dec. 31, 2023	$ 169,911					$ 8	352,908	0	(183,005)
Temporary equity, ending balance (in share) at Dec. 31, 2023		0	0	0	0
Temporary equity, ending balance at Dec. 31, 2023		$ 0	$ 0	$ 0	$ 0
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

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Operating Activities:
Net loss	$ (19,557)	$ (19,605)	$ (81,172)	$ (58,032)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash lease expense	1,203	432	3,610	1,396
Stock-based compensation expense	865	349	2,381	1,140
Depreciation expense	726	809	3,630	2,511
Non-cash interest expense			0	118
Net amortization of premiums and discounts on investments	0	(68)	(80)	(145)
Loss on disposal of property and equipment			65	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(290)	(341)	559	(28)
Accounts payable	(2,585)	549	3,100	1,755
Accrued expenses and other current liabilities	(5,048)	(1,117)	1,270	(39)
Operating lease liabilities	2,709	(695)	1,341	(2,198)
Other non-current assets	86	(2)	(1,987)	(123)
Net cash used in operating activities	(21,891)	(19,689)	(67,283)	(53,645)
Investing Activities:
Purchases of investments				(37,213)
Proceeds from maturities of investments	0	17,000	19,000	53,485
Purchases of property and equipment	(5,611)	(1,513)	(7,836)	(5,136)
Advance payments for property and equipment not yet received				(76)
Net cash provided by (used in) investing activities	(5,611)	15,487	11,164	11,060
Financing Activities:
Proceeds from exercises of stock options	139	51	702	62
Proceeds from the issuance of common stock in pre-closing financing			117,250	0
Cash acquired in connection with the reverse recapitalization			36,600	0
Payment of reverse recapitalization transaction costs			(12,247)	0
Other financing activities, net	0	(150)	(2)	(44)
Net cash provided by financing activities	139	45,401	187,761	18
Net increase (decrease) in cash, cash equivalents and restricted cash	(27,363)	41,199	131,642	(42,567)
Cash, cash equivalents and restricted cash, beginning of period	173,119	41,477	41,477	84,044
Cash, cash equivalents and restricted cash, end of period	145,756	82,676	173,119	41,477
Non-cash investing and financing activities:
Property and equipment capitalized under tenant improvement allowance	2,555	0	2,271	0
Purchases of property and equipment in accounts payable and accrued expenses	1,942	1,753	1,983	402
Financing costs in accounts payable and accrued expenses	0	42
Operating Lease Liabilities Arising From Right-Of-Use Assets			26,777	5,629
Operating lease liabilities arising from lease modification			1,959	0
Supplemental cash flow information:
Cash paid for income taxes	17	0	27	11
Cash paid for operating lease liabilities	828	744	2,978	2,335
Series B-2 Preferred Stock [Member]
Financing Activities:
Proceeds from Series B-2 convertible preferred stock, net of issuance costs	$ 0	$ 45,500	$ 45,458	$ 0

The Company and Liquidity

The Company and Liquidity	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 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 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 Pre-Closing 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 tax-free 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 non-operating pre-combination Reverse Stock Split and Exchange Ratio In connection with, and prior to the completion of, the Merger, Frequency effected a 1-for-50 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.	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 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 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, 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, 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. Shares of Legacy Korro common stock issued pursuant to the Pre-Closing 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 Legacy Korro’s pre-closing tax-free Upon closing of the Merger, the Company assumed the Legacy Korro 2019 Stock Incentive 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 non-operating pre-combination Reverse Stock Split and Exchange Ratio In connection with, and prior to the completion of, the merger, Frequency effected a one-for-fifty 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 December 31, 2023 of $166.1 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 December 31, 2023, the Company had an accumulated deficit of $183.0 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.

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 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 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. 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 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 and other segment items and identifying the Chief Operating Decision Maker interim	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. 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 CVR liability and the incremental borrowing rate for determining lease liabilities and right-of-use Fair Value of Financial Instruments ASC Topic 820, Fair Value Measurement ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier fair value hierarchy that distinguishes between the following: • Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities. • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. • Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Cash Equivalents Cash equivalents are highly-liquid investments that are readily convertible into cash with original maturities of three months or less when purchased. These assets include investments in money market funds that invest in U.S. Treasury obligations. Cash equivalents are reflected at fair value based on quoted market prices, as further described in Note 3, “Fair Value Measurements”. Investments Investments consist of securities with original maturities greater than three months when purchased. Short-term investments consist of investments that are available for use in current operations. Long-term investments consist of investments with maturities of greater than one year that are not available for use in current operations. The Company did not maintain any long-term investments as of December 31, 2023 or 2022. The Company classifies all of its investments as available-for-sale available-for-sale The Company reviews its investment portfolio to identify and evaluate investments that have indicators of possible other-than-temporary impairment. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer and the Company’s intent and ability to hold the investment for a period of time sufficient to allow for any anticipated recovery in market value. Property and Equipment Property and equipment are recorded at cost and consists of laboratory equipment, furniture and office equipment, computer equipment, leasehold improvements, and construction in progress. The Company capitalizes property and equipment that is acquired for research and development activities and that has alternative future use. Expenditures for repairs and maintenance are recorded to expense as incurred, whereas major betterments are capitalized as additions to property and equipment. Property and equipment not yet placed into service is capitalized as construction in progress and is depreciated once placed into service. Leasehold improvements are depreciated over the lesser of their useful lives or the term of the lease. Depreciation, including depreciation for assets recorded under finance leases, is calculated over the estimated useful lives of the assets using the straight-line method. Impairment of Long-lived Assets The Company reviews long-lived assets when events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. Recoverability is measured by comparing the carrying value of the asset to the future undiscounted cash flows from the use and eventual disposition of the asset. If an asset is considered to be impaired, the impairment loss to be recognized is measured as the amount by which the carrying value of the asset exceeds its fair value. Leases Under ASC Topic 842, Leases right-of-use Right-of-use right-of-use The Company has elected to account for the lease and non-lease Preferred Stock The Company applies the guidance of ASC Topic 480, Distinguishing Liabilities from Equity (“ASC 480”), when determining the classification and measurement of its preferred stock. Preferred stock subject to mandatory redemption (if any) are classified as liability instruments and are measured at fair value. The Company classifies contingently redeemable preferred stock (if any), which includes preferred stock that features redemption rights that are either within the control of the holder or subject to redemption upon the occurrence of uncertain events not solely within the Company’s control, as temporary equity. At all other times, the Company classifies its preferred stock in stockholders’ equity (deficit). Prior to the merger, Legacy Korro classified its convertible preferred stock as temporary equity due to terms that allowed for redemption of the shares upon the occurrence of a contingent event that was not solely within the Legacy Korro’s control. Legacy Korro did not accrete the carrying values of the preferred stock to the redemption values since the contingent event was not considered probable. Research and Development Expenses Expenditures relating to research and development are expensed as incurred. Research and development expenses include external expenses incurred under arrangements with third parties, academic and non-profit non-cash in-process Non-refundable As part of the process of preparing the consolidated financial statements, the Company is required to estimate its accrued research and development expenses as of each balance sheet date. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. This process involves reviewing open contracts and purchase orders, communicating with internal personnel to identify services that have been performed on the Company’s behalf and estimating the level of service performed and the associated cost incurred for the service when the Company has not yet been invoiced or otherwise notified of the actual cost. The Company periodically confirms the accuracy of its estimates with its service providers and makes adjustments if necessary. The majority of the Company’s service providers invoice monthly in arrears for services performed or when contractual milestones are met. The financial terms of agreements with these service providers are subject to negotiation, vary from contract-to-contract Intellectual Property Expenses The Company expenses legal costs related to patent applications as they are incurred. Such costs are classified as general and administrative expenses within the consolidated statements of operations and comprehensive loss. Stock-based Compensation The Company accounts for stock-based payments in accordance with ASC Topic 718, Compensation—Stock Compensation non-employees non-employees, Prior to the merger, due to the absence of an active market for Legacy Korro’s common stock, Legacy Korro and its Board were required to determine the fair value of Legacy Korro’s common stock at the time of each grant of a stock-based award. Legacy Korro estimated the grant date fair value of its common stock using an appropriate valuation methodology, in accordance with the framework of the American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation Each valuation methodology includes estimates and assumptions that required the Legacy Korro’s judgment. These estimates and assumptions include a number of objective and subjective factors, including external market conditions, guideline public company information, the prices at which Legacy Korro sold convertible preferred stock to third parties in arms’ length transactions, the rights and preferences of securities senior to Legacy Korro’s common stock at the time and the likelihood of achieving a liquidity event such as an initial public offering or sale. Significant changes to the assumptions used in the valuations could result in different fair values of stock options and restricted stock at each valuation date, as applicable. In addition to the grant date fair value of the Company’s common stock, the Black-Scholes option pricing model requires the input of certain subjective assumptions, including (i) the calculation of expected term of the stock-based payment, (ii) the risk-free interest rate, (iii) the expected stock price volatility and (iv) the expected dividend yield. The Company uses the simplified method as proscribed by SEC Staff Accounting Bulletin No. 107 to calculate the expected term for stock options granted to employees as the Company does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. The Company determines the risk-free interest rate based on a treasury instrument whose term is consistent with the expected term of the stock options. Due to the lack of Company-specific historical and implied volatility data, the Company bases its estimates of expected volatility on the historical volatility of a group of publicly-traded companies with similar characteristics to itself, including stage of product development and therapeutic focus within the life sciences industry. The expected volatility has been determined using a weighted-average of the historical volatility measures of this group of companies. The Company expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. Historical volatility is calculated over a period of time commensurate with the expected term of the stock-based payment. The Company uses an assumed dividend yield of zero as the Company has never paid dividends on its common stock, nor does it expect to pay dividends on its common stock in the foreseeable future. The Company accounts for forfeitures of all stock-based payments when such forfeitures occur. Income Taxes Income taxes are recorded in accordance with ASC Topic 740, Income Taxes The Company accounts for uncertain tax positions using a more-likely-than-not Interest and penalty charges, if any, related to income taxes would be classified as a component of the “Provision for income taxes” in the consolidated statements of operations and comprehensive loss. Net Loss per Share Net loss per share attributable to common stockholders is calculated using the two-class Diluted net loss per share is computed using the more dilutive of (a) the two-class if-converted Concentration of Credit Risk and Off-Balance Financial instruments that potentially expose the Company to concentrations of credit risk primarily consist of cash, cash equivalents and investments. Cash balances are deposited with federally-insured financial institutions in the United States and may, at times, exceed federally-insured limits. The Company maintains its cash, cash equivalents and investments with high-quality financial institutions and, consequently, the Company believes that such funds are subject to minimal credit risk. The Company’s cash equivalents are comprised of money market funds that are invested in U.S. Treasury and government agency obligations. The Company’s investments are comprised of commercial paper and government securities. Credit risk in these securities is reduced as a result of the Company’s investment policy to limit the amount invested in any single issuer and to only invest in securities of a high credit quality. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and to process its product candidates for its development programs. These programs could be adversely affected by a significant interruption in the manufacturing process or supply chain. The Company has no significant off-balance Emerging growth company status The Company qualifies as an “emerging growth company” (“EGC”), as defined in the Jumpstart Our Business Startups Act (“JOBS Act”), and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not EGCs. The Company may take advantage of these exemptions until it is no longer an EGC under Section 107 of the JOBS Act, which provides that an EGC can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to avail itself of the extended transition period and, therefore, while the Company is an EGC it will not be subject to new or revised accounting standards the same time that they become applicable to other public companies that are not EGCs, unless it chooses to early adopt a new or revised accounting standard. As a result of this election, the consolidated financial statements may not be comparable to companies that comply with public company FASB standards’ effective dates. Recently Adopted Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments 2018-19, 2019-04, 2019-05, 2019-10, 2019-11, 2020-03, 2022-02 2016-13”). 2016-13 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 FASB issued Accounting Standards Update (“ASU”) 2023-07, Segment Reporting (Topic 280), Improvements to Reportable Segment Disclosures

Reverse Merger

Reverse Merger	12 Months Ended
	Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]
Reverse Merger	3. Reverse Merger As described in Note 1, Legacy Korro merged with Frequency on November 3, 2023. The merger was accounted for as a reverse recapitalization with Legacy Korro as the accounting acquirer. The primary pre-combination As part of the reverse recapitalization, the Company acquired $36.6 million of cash and cash equivalents. The Company also obtained prepaids and other assets of $1.9 million and assumed accounts payable and accrued expenses of $5.4 million. Frequency’s development programs had ceased prior to the merger and were deemed to be de minimis in value at the transaction date. In addition, the Company recognized $0.3 million in share-based compensation expense as a result of the acceleration of vesting of stock options, performance stock units and restricted stock units at the time of merger. This amount was recorded in general and administrative expense in the accompanying consolidated statements of operations and comprehensive loss for the year ended December 31, 2023. The company recognized $2.5 million in personnel cost related to the severance payment to Frequency employees and this amount was recorded in general and administrative expense in the accompanying consolidated statements of operations and comprehensive loss for the year ended December 31, 2023. The Company also incurred transaction costs of $12.2 million and this amount is recorded in additional paid-in

Fair Value Measurements

Fair Value Measurements	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 Significant Significant 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 Significant Significant 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 non-current derivative asset 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.	4. Fair Value Measurements The Company measures the fair value of money market funds based on quoted prices in active markets for identical securities. Investments also include commercial paper and government securities that are valued either based on recent trades of securities in inactive markets or based on quoted market prices of similar instruments and other significant inputs derived from or corroborated by observable market data. 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 measured at fair value on a recurring basis as of December 31, 2023 were as follows (in thousands): Total Quoted Prices Significant Significant Money market funds, included in cash and cash equivalents $ 158,706 $ 158,706 $ —  $ —  MS APA asset $ 1,448 $ —  $ —  $ 1,448 Total $ 160,154 $ 158,706 $ —  $ 1,448 Assets measured at fair value on a recurring basis as of December 31, 2022 were as follows (in thousands): Total Quoted Prices Significant Significant Money market funds, included in cash and cash equivalents $ 14,904 $ 14,904 $ —  $ —  Short-term investments: Commercial paper $ 14,935 —  14,935 —  Government securities $ 3,980 —  3,980 —  Total $ 33,819 $ 14,904 $ 18,915 $ —  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 the Company’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 non-current derivative asset 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 year ended December 31, 2023, the aggregate change in fair value of the CVR liability and MS APA asset was $0.08 million. There were no liabilities measured at fair value on a recurring basis as of December 31, 2022. There were no changes in valuation techniques, nor were there any transfers among the fair value hierarchy levels during the years ended December 31, 2023 or 2022.

Investments

Investments	3 Months Ended
	Mar. 31, 2024
Investments [Abstract]
Investments	5. Investments The Company did not have investments as of December 31, 2023. Short-term investments as of December 31, 2022 were comprised as follows (in thousands): Amortized Unrealized Unrealized Fair Commercial paper $ 14,935 $ —  $ —  $ 14,935 Government securities 3,985 —  (5 ) 3,980 Total $ 18,920 $ —  $ (5 ) $ 18,915 As of December 31, 2022, the aggregate fair value of securities that were in an unrealized loss position for less than twelve months was $4.0 million. As of December 31, 2022, the Company held no securities that were in an unrealized loss position for more than twelve months. As of December 31, 2022, the Company did not intend to sell, and would not be more likely than not required to sell, the securities in an unrealized loss position before recovery of their amortized cost bases. Furthermore, the Company determined that there was no material change in the credit risk of these securities. As a result, the Company determined it did not hold any securities with any other-than-temporary impairment as of December 31, 2022.

Restricted Cash

Restricted Cash	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 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, December 31, Cash and cash equivalents $ 138,804 $ 166,150 Restricted cash 6,952 6,969 Cash, cash equivalents and restricted cash $ 145,756 $ 173,119	6. Restricted Cash As of December 31, 2023, the Company maintained current restricted cash of $3.6 million and non-current non-current The following table provides a reconciliation of cash, cash equivalents and restricted cash as of December 31, 2023 and 2022 that sums to the total of the same amounts shown in the consolidated statements of cash flows (in thousands): December 31, 2023 2022 Cash and cash equivalents $ 166,150 $ 36,333 Restricted cash 6,969 5,144 Cash, cash equivalents and restricted cash $ 173,119 $ 41,477

Property and Equipment , Net

Property and Equipment , Net	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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, December 31, 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.	7. Property and Equipment, Net Property and equipment, net, as of December 31, 2023 and 2022 was comprised as follows (in thousands): December 31, Estimated Useful Life (in Years) 2023 2022 Laboratory equipment 5 $ 11,187 $ 8,441 Furniture and office equipment 4 481 477 Computer equipment 3 241 213 Leasehold improvements Shorter of useful life or remaining lease term 3,356 2,941 Construction in progress 8,242 2,049 Total property and equipment, gross 23,507 14,121 Less: accumulated depreciation (7,842 ) (4,255 ) Total property and equipment, net $ 15,665 $ 9,866 As of December 31, 2023 and 2022, the Company had construction in progress of $8.2 million and $2.0 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 years ended December 31, 2023 and 2022 was $3.6 million and $2.5 million, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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, December 31, 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	8. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities as of December 31, 2023 and 2022 were comprised as follows (in thousands): December 31, 2023 2022 Employee compensation and benefits $ 7,896 $ 2,624 External research and development services 1,255 274 Other operating expenses 447 207 CVR liability, current 395 —  Professional fees 219 70 Total accrued expenses and other current liabilities $ 10,212 $ 3,175

Common Stock

Common Stock	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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, December 31, 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	9. 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 December 31, 2023, 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 set forth in Note 10. 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 December 31, 2023, no dividends had been declared. As of December 31, 2023 and 2022, the Company had reserved for future issuance the following number of shares of common stock: December 31, 2023 2022 Conversion of outstanding Legacy Korro Series Seed Preferred Stock —  684,739 Conversion of outstanding Legacy Korro Series A Preferred Stock —  2,029,666 Conversion of outstanding Legacy Korro Series B-1 —  1,104,178 Conversion of outstanding Legacy Korro Series B-2 —  223,417 Future issuances of Legacy Korro Series B-2 —  813,239 Vesting of restricted common stock —  2,902 Exercises of outstanding stock options 1,323,151 461,128 Exercise of outstanding warrant 8,049 8,049 Future issuances under 2019 Stock Incentive Plan —  159,890 Future issuances under 2023 Stock Incentive Plan 255,694 —  Future issuances under 2023 ESPP Plan 88,502 —  Total reserved for future issuance 1,675,396 5,487,208

Preferred Stock

Preferred Stock	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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	10. Preferred Stock As of December 31, 2023, 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 the 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 As of December 31, 2022, Legacy Korro 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. As of December 31, 2022, Legacy Korro Preferred Stock authorized, issued and outstanding consisted of the following (in thousands, except share amounts): December 31, 2022 Preferred Preferred Carrying Liquidation Common Legacy Korro Series Seed Preferred Stock 684,739 684,739 15,924 $ 16,115 684,739 Legacy Korro Series A Preferred Stock 2,029,666 2,029,666 77,736 91,500 2,029,666 Legacy Korro Series B-1 1,104,178 1,104,178 57,703 58,000 1,104,178 Legacy Korro Series B-2 1,036,656 223,417 12,500 12,500 223,417 Total 4,855,239 4,042,000 $ 163,863 $ 178,115 4,042,000 Rights, Preferences and Privileges of Legacy Korro Preferred Stock The Legacy Korro Preferred Stock as of December 31, 2022 had the following rights and preferences. In the discussion below, the Legacy Korro Series Seed Preferred Stock and the Legacy Korro Series A Preferred Stock, the Legacy Korro Series B-1 B-2 Conversion Each share of Legacy Korro Preferred Stock were convertible, at the option of the holder thereof, at any time and from time to time, and without the payment of additional consideration by the holder thereof, into such number of fully paid and non-assessable B-1 B-2 The Legacy Korro Preferred Stock was subject to mandatory conversion upon the closing of a sale of common stock to the public at a price of at least $111.90 per share (subject to appropriate adjustment in the event of a stock dividend, stock split, combination or other similar recapitalization) in a firm-commitment underwritten public offering resulting in at least $75.0 million of gross proceeds to Legacy Korro. The Legacy Korro Preferred Stock was also subject to mandatory conversion upon the vote or written consent of the holders of at least 66% of the then-outstanding shares of Legacy Korro Preferred Stock, voting as a single class on an as-converted Dividends The holders of Legacy Korro Preferred Stock, in preference to common stockholders, were entitled to receive, when, as and if declared by the Company’s Board of Directors, dividends at a rate of 8% annually. Dividends on Legacy Korro Preferred Stock were non-cumulative as-converted Liquidation Preference In the event of any voluntary or involuntary liquidation, dissolution, winding up or deemed liquidation event of Legacy Korro, the holders of shares of Legacy Korro Preferred Stock then outstanding were entitled to be paid out of the assets of Legacy Korro available for distribution to its stockholders before any payment shall be made to the holders of common stock, an amount equal to the original issue price per share plus any dividends declared but unpaid thereon. For clarity, the original issue price of the Legacy Korro Series Seed 1 Preferred Stock and Legacy Korro Series Seed 2 Preferred Stock was $20.13 per share, the original issue price of the Legacy Korro Series Seed 3 Preferred Stock was $26.16 per share, the original issue price of the Legacy Korro Series A Preferred Stock was $45.08 per share, the original issue price of the Legacy Korro Series B-1 B-2 If upon any voluntary or involuntary liquidation, dissolution, winding up or deemed liquidation event of Legacy Korro, the assets of Legacy Korro available for distribution were insufficient to pay the holders of Legacy Korro Preferred Stock the full amount to which they were entitled, the holders of shares of Legacy Korro Preferred Stock shall share ratably in any distribution of the assets available for distribution in proportion to the respective amounts that would otherwise be payable in respect of the shares held by them upon such distribution if all amounts payable on or with respect to such shares were paid in full. Redemption The Legacy Korro Preferred Stock was contingently redeemable upon the occurrence of a deemed liquidation event, which included a merger or a sale of substantially all of the assets of Legacy Korro. As of December 31, 2022, a deemed liquidation event was not considered to be probable. Voting Rights The holders of Legacy Korro Preferred Stock were entitled to vote based on the number of common shares that their preferred shares converted into on as-converted The holders of record of the shares of the Legacy Korro Series Seed Preferred Stock, voting exclusively and as a separate class on an as-converted as-converted as-converted

Stock-based Compensation

Stock-based Compensation	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 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 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 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- Weighted- Aggregate 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.	11. Stock-based Compensation 2023 Stock Option and Incentive Plan In November 2023, the Company’s board of directors adopted the Company’s 2023 Stock Option and Incentive Plan (the “2023 Plan”). The 2023 Plan was approved by the Company’s stockholders in November 2023 and became effective in November 2023 in connection with completion of the Merger. The 2023 Plan initially reserved 885,028 shares for the issuance of stock awards. In addition, the number of shares of common stock available for issuance under the 2023 Plan will be automatically increased on the first day of each calendar year during the ten-year term of the 2023 Plan, beginning with January 1, 2024 and ending with January 1, 2033, by the amount equal to 5% of the outstanding number of shares of the Company’s common stock on December 31 of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. Awards granted under the 2023 Plan expire no later than ten years from the date of grant. For stock options, the option price shall generally not be less than 100% of the fair market value on the day of grant. As of December 31, 2023, there were in aggregate 889,922 shares of common stock reserved for issuance and 255,694 shares available to grant under the 2023 Plan. 2019 Stock Incentive Plan In January 2019, the Legacy Korro’s Board of Directors adopted the 2019 Stock Incentive Plan (as amended from time to time, the “Legacy Korro Plan”). The Legacy Korro Plan provides for the grant of stock options, stock awards and restricted stock units to employees, members of the Company’s Board of Directors and non-employee In connection with the merger, each option to purchase shares of Legacy Korro common stock that was outstanding and unexercised under the Legacy Korro Plan immediately prior to the Effective Time, whether or not vested, was converted into an option to purchase shares of the Company’s common stock and became eligible to be registered on Form S-8. The number of shares of common stock reserved for issuance pursuant to outstanding awards under the Legacy Korro Plan as of December 31, 2023 and 2022 was 600,977 shares and 683,156 shares, respectively. The number of shares available for grant under the Legacy Korro Plan was 0 and 159,890 at December 31, 2023 and 2022, respectively. Upon completion of the Merger and effectiveness of the 2023 Plan, the Company ceased granting awards under the Legacy Korro Plan. 2014 Stock Incentive Plan On November 13, 2014, Frequency adopted the 2014 Stock Incentive Plan (as amended from time to time,the “2014 Plan”). All of Frequency’s employees, officers, directors, and consultants were eligible to be granted options to purchase common shares, restricted stock units and restricted stock under the terms of the 2014 Plan. Frequency, reserved an aggregate of 424,853 shares of common stock for issuance under the 2014 Plan. The number of shares under the 2014 Plan subject to outstanding awards as of the effective date of the 2023 Plan that are subsequently canceled, forfeited or repurchased by the Company will be added to the shares reserved under the 2023 Plan. As of December 31, 2023, there were no shares of common stock available for future grants under the 2014 Plan and there were 36,455 shares subject to outstanding option awards under the 2014 Plan. 2019 Incentive Award Plan On September 17, 2019, Frequency’s board of directors and on September 19, 2019, its stockholders approved and adopted the 2019 Incentive Award Plan (the “2019 Plan”). Under the 2019 Plan, Frequency may grant stock options, stock appreciation rights, restricted stock, restricted stock units, and other stock and cash-based awards to individuals who are then employees, officers, directors or consultants of the Company, and employees and consultants of the Company’s subsidiaries. A total of 154,032 shares of common stock were approved to be initially reserved for issuance under the 2019 plan. The number of shares under the 2019 Plan subject to outstanding awards as of the effective date of the 2023 Plan that are subsequently canceled, forfeited or repurchased by the Company will be added to the shares reserved under the 2023 Plan. Upon completion of the Merger and effectiveness of the 2023 Plan, the Company ceased granting awards under the 2019 Plan. Accordingly, as of December 31, 2023, there were no shares of common stock available for future grants under the 2019 Plan and there were 51,491 shares subject to outstanding option awards under the 2019 Plan. All stock option grants are non-statutory Stock-based Compensation Expense Total stock-based compensation expense recognized in the consolidated statements of operations and comprehensive loss for the years ended December 31, 2023 and 2022 was as follows (in thousands): Year Ended December 31, 2023 2022 Research and development $ 759 $ 243 General and administrative 1,622 897 Total stock-based compensation expense $ 2,381 $ 1,140 Restricted Common Stock Activity Prior to the adoption of the 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 The following table summarizes Legacy Korro restricted common stock activity during the year ended December 31, 2023: Shares Weighted- Unvested as of December 31, 2022 2,902 $ 0.59 Granted —  $ —  Vested 2,902 $ 0.23 Repurchased —  $ —  Unvested as of December 31, 2023 —  $ —  The aggregate fair value of Legacy Korro restricted common stock that vested during the years ended December 31, 2023 and 2022, based upon the fair value of the underlying restricted common stock on the day of vesting, was $0.1 million and $0.5 million, respectively. Stock Option Activity The fair value of stock options granted during the years ended December 31, 2023 and 2022 was calculated on the date of grant using the following weighted-average assumptions: Year Ended December 31, 2023 2022 Risk-free interest rate 4.3 % 2.5 % Expected dividend yield — % — % Expected term (in years) 6.0 6.0 Expected volatility 72.8 % 72.9 % Using the Black-Scholes option pricing model, the weighted-average grant date fair value of stock options granted during the years ended December 31, 2023 and 2022 was $23.49 and $14.95 per share, respectively. The following table summarizes changes in stock option activity during the year ended December 31, 2023 (in thousands, except per share amounts): Options Weighted- Weighted- Aggregate Outstanding at December 31, 2022 461,100 $ 14.95 8.0 $ 2,953 Assumed in reverse recapitalization 93,153 $ 121.91 Granted 835,839 $ 17.33 Exercised (39,395 ) $ 16.41 Forfeited (20,091 ) $ 20.07 Cancelled (7,455 ) $ 18.85 Outstanding as of December 31, 2023 1,323,151 $ 23.49 8.4 $ 36,310 Exercisable at December 31, 2023 368,036 $ 34.60 6.7 $ 8,457 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 years ended December 31, 2023 and 2022 was $0.3 million and $0.1 million, respectively. As of December 31, 2023, there was unrecognized stock-based compensation expense related to unvested stock options of $10.1 million, which the Company expects to recognize over a weighted-average period of approximately 3.1 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes The provision for income taxes for the years ended December 31, 2023 and 2022 was comprised as follows (in thousands): Year Ended 2023 2022 Current taxes: Federal $ —  $ —  State 27 10 Total current taxes 27 10 Deferred taxes: Federal —  —  State —  —  Total deferred taxes —  —  Total provision for income taxes $ 27 $ 10 A reconciliation of the federal statutory income tax rate to the Company’s effective tax rate is as follows: December 31, 2023 2022 Income tax computed at federal statutory rate 21.0 % 21.0 % State taxes, net of federal benefit 7.1 % 6.1 % Tax credit carryforwards 3.6 % 6.2 % Permanent items (1.0 )% (0.2 )% Change in valuation allowance (30.8 )% (32.7 )% Other 0.1 % (0.4 )% Effective tax rate — % — % The principal components of the Company’s deferred tax assets and liabilities as of December 31, 2023 and 2022 were comprised as follows (in thousands): December 31, 2023 2022 Deferred tax assets: Net operating loss carryforwards $ 80,385 $ 19,580 Tax credit carryforwards 22,474 5,477 Capitalized research and development 34,395 10,279 Stock-based compensation 4,494 319 Amortization 1,138 —  Operating lease liability 9,052 855 Accrued expenses and other temporary differences 1,543 707 Total deferred tax assets 153,481 37,217 Less: valuation allowance (145,766 ) (36,094 ) Net deferred tax assets 7,715 1,123 Deferred tax liabilities: Operating right-of-use (7,401 ) (552 ) Depreciation (314 ) (571 ) Total deferred tax liabilities (7,715 ) (1,123 ) Net deferred taxes $ —  $ —  As of December 31, 2023, the Company had federal and state net operating loss (“NOL”) carryforwards of $302.5 million and $266.3 million, respectively. Under current law, the Company’s federal NOLs generated in taxable years ending after December 31, 2017, may be carried forward indefinitely, but the deductibility of such federal NOLs is limited to 80% of the Company’s taxable income annually for tax years beginning after December 31, 2018. Federal NOLs generated in taxable years ending on or prior to December 31, 2017, however, have a 20-year State NOLs expire at various dates from 2035 through 2043. As of December 31, 2023, the Company had federal research and development tax credit carryforwards of $15.6 million that expire at various dates from 2036 through 2043. In addition, as of December 31, 2023, the Company had state research and development tax credit carryforwards of $8.7 million that expire at various dates from 2032 through 2038. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which primarily pertain to NOL carryforwards, tax credit carryforwards and capitalized research and development. The Company has determined that it is more likely than not that it will not realize the benefits of its deferred tax assets, and as a result, a valuation allowance of $145.8 million has been established at December 31, 2023. The increase in the valuation allowance of $109.7 million during the year ended December 31, 2023 was primarily due to the additional operating loss generated by the Company and the combination of deferred tax assets as a result of the Merger. NOL and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50% as defined under Sections 382 and 383 in the IRC. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the Company’s value immediately prior to the ownership change. As a result of ownership changes in the Company from its inception through December 31, 2023, the Company’s NOL and tax credit carryforwards allocable to the periods preceding each such ownership change could be subject to limitations under IRC Section 382, however the Company has not yet completed an IRC Section 382 study. The Company had no unrecognized tax benefits as of either December 31, 2023 or 2022. The Company has not conducted a study of its research and development credit carryforwards generated during any year. This study, once completed, may result in an adjustment to the Company’s research and development credit carryforwards. However, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credit carryforwards, and if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the consolidated statements of operations and comprehensive loss if an adjustment were required. The Company files income tax returns in the United States federal tax jurisdiction and the Connecticut and Massachusetts state tax jurisdictions. Because the Company is in a loss carryforward position, it is generally subject to examination by federal and state tax authorities for all tax years in which a loss carryforward is available. As of December 31, 2023, the Company has not incurred any material interest or penalty charges.

Genevant Agreement

Genevant Agreement	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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: • 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. • The Company will reimburse Genevant for certain out-of-pocket • 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. • Genevant is eligible to receive royalties at percentage rates in the mid-single-digits, 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.	13. 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: • 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. • The Company will reimburse Genevant for certain out-of-pocket • 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. • Genevant is eligible to receive royalties at percentage rates in the mid-single-digits, As of December 31, 2023, no milestones have been achieved and the Company has recorded payments to Genevant of $1.2 million within research and development expense in the consolidated statement of operations during the year ended December 31, 2023.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 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 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 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 right-of-use 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 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) FX-322 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.	14. 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 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 Future minimum lease payments for all three leases, net of $10.8 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 December 31, 2023 were as follows (in thousands): As of December 31, 2023 2024 (7,227 ) 2025 6,247 2026 7,341 2027 7,557 Thereafter 52,498 Total Future Minimum Leases Payments 66,416 Less: Interest 33,209 Present Value of Operating Lease Liabilities 33,207 As of December 31, 2023, the weighted average remaining lease term was 10.0 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 right-of-use 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): Year Ended December 31, 2023 2022 Operating lease costs $ 5,948 $ 1,675 Variable Lease Costs 1,211 732 Short-term Lease Costs 921 626 Total Lease Costs $ 8,080 $ 3,033 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) FX-322 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.

401(k) Savings Plan

401(k) Savings Plan	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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.	15. 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 $0.5 million and $0.2 million of matching contributions to the 401(k) Plan during the year ended December 31, 2023 and December 31, 2022, respectively.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2023
Related Party Transactions [Abstract]
Related Party Transactions	16. Related Party Transactions As a result of Atlas’ ownership of the Legacy Korro’s Series Seed Preferred Stock, Series A Preferred Stock and Series B Preferred Stock which was exchanged in the Merger for the Company’s common stock, Atlas represents an affiliate of the Company. During the years ended December 31, 2023 and 2022, the Company incurred no expense and expenses of less than $0.1 million, respectively, related to consulting services provided by an affiliate of Atlas. As of December 31, 2023 and 2022, the Company had no amounts due to this affiliate.

Net Loss per Share

Net Loss per Share	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 2024 2023 Legacy Korro Series Seed Preferred Stock —  684,739 Legacy Korro Series A Preferred Stock —  2,029,666 Legacy Korro Series B-1 —  1,104,178 Legacy Korro Series B-2 —  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 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 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 )	17. 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: Year Ended December 31, 2023 2022 Legacy Korro Series Seed Preferred Stock —  684,739 Legacy Korro Series A Preferred Stock —  2,029,666 Legacy Korro Series B-1 —  1,104,178 Legacy Korro Series B-2 —  223,417 Unvested Legacy Korro restricted common stock —  2,902 Outstanding Legacy Korro stock options 1,323,151 461,100 Outstanding Legacy Korro warrant 8,049 8,049 Total 1,331,200 4,514,051 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): Years Ended December 31, 2023 2022 Numerator: Net loss $ (81,172 ) $ (58,032 ) Denominator: Weighted-average number of shares outstanding, basic and diluted 1,529,321 255,175 Net loss per share, basic and diluted $ (53.08 ) $ (227.42 )

Subsquent Events

Subsquent Events	12 Months Ended
	Dec. 31, 2023
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.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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	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 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. 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.	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	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 CVR liability and the incremental borrowing rate for determining lease liabilities and right-of-use
Fair Value of Financial Instruments		Fair Value of Financial Instruments ASC Topic 820, Fair Value Measurement ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three-tier fair value hierarchy that distinguishes between the following: • Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities. • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. • Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.
Cash Equivalents		Cash Equivalents Cash equivalents are highly-liquid investments that are readily convertible into cash with original maturities of three months or less when purchased. These assets include investments in money market funds that invest in U.S. Treasury obligations. Cash equivalents are reflected at fair value based on quoted market prices, as further described in Note 3, “Fair Value Measurements”.
Investments		Investments Investments consist of securities with original maturities greater than three months when purchased. Short-term investments consist of investments that are available for use in current operations. Long-term investments consist of investments with maturities of greater than one year that are not available for use in current operations. The Company did not maintain any long-term investments as of December 31, 2023 or 2022. The Company classifies all of its investments as available-for-sale available-for-sale The Company reviews its investment portfolio to identify and evaluate investments that have indicators of possible other-than-temporary impairment. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer and the Company’s intent and ability to hold the investment for a period of time sufficient to allow for any anticipated recovery in market value.
Property and Equipment		Property and Equipment Property and equipment are recorded at cost and consists of laboratory equipment, furniture and office equipment, computer equipment, leasehold improvements, and construction in progress. The Company capitalizes property and equipment that is acquired for research and development activities and that has alternative future use. Expenditures for repairs and maintenance are recorded to expense as incurred, whereas major betterments are capitalized as additions to property and equipment. Property and equipment not yet placed into service is capitalized as construction in progress and is depreciated once placed into service. Leasehold improvements are depreciated over the lesser of their useful lives or the term of the lease. Depreciation, including depreciation for assets recorded under finance leases, is calculated over the estimated useful lives of the assets using the straight-line method.
Impairment of Long-lived Assets		Impairment of Long-lived Assets The Company reviews long-lived assets when events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. Recoverability is measured by comparing the carrying value of the asset to the future undiscounted cash flows from the use and eventual disposition of the asset. If an asset is considered to be impaired, the impairment loss to be recognized is measured as the amount by which the carrying value of the asset exceeds its fair value.
Leases		Leases Under ASC Topic 842, Leases right-of-use Right-of-use right-of-use The Company has elected to account for the lease and non-lease
Preferred Stock		Preferred Stock The Company applies the guidance of ASC Topic 480, Distinguishing Liabilities from Equity (“ASC 480”), when determining the classification and measurement of its preferred stock. Preferred stock subject to mandatory redemption (if any) are classified as liability instruments and are measured at fair value. The Company classifies contingently redeemable preferred stock (if any), which includes preferred stock that features redemption rights that are either within the control of the holder or subject to redemption upon the occurrence of uncertain events not solely within the Company’s control, as temporary equity. At all other times, the Company classifies its preferred stock in stockholders’ equity (deficit). Prior to the merger, Legacy Korro classified its convertible preferred stock as temporary equity due to terms that allowed for redemption of the shares upon the occurrence of a contingent event that was not solely within the Legacy Korro’s control. Legacy Korro did not accrete the carrying values of the preferred stock to the redemption values since the contingent event was not considered probable.
Research and Development Expenses		Research and Development Expenses Expenditures relating to research and development are expensed as incurred. Research and development expenses include external expenses incurred under arrangements with third parties, academic and non-profit non-cash in-process Non-refundable As part of the process of preparing the consolidated financial statements, the Company is required to estimate its accrued research and development expenses as of each balance sheet date. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. This process involves reviewing open contracts and purchase orders, communicating with internal personnel to identify services that have been performed on the Company’s behalf and estimating the level of service performed and the associated cost incurred for the service when the Company has not yet been invoiced or otherwise notified of the actual cost. The Company periodically confirms the accuracy of its estimates with its service providers and makes adjustments if necessary. The majority of the Company’s service providers invoice monthly in arrears for services performed or when contractual milestones are met. The financial terms of agreements with these service providers are subject to negotiation, vary from contract-to-contract
Intellectual Property Expenses		Intellectual Property Expenses The Company expenses legal costs related to patent applications as they are incurred. Such costs are classified as general and administrative expenses within the consolidated statements of operations and comprehensive loss.
Stock-based Compensation		Stock-based Compensation The Company accounts for stock-based payments in accordance with ASC Topic 718, Compensation—Stock Compensation non-employees non-employees, Prior to the merger, due to the absence of an active market for Legacy Korro’s common stock, Legacy Korro and its Board were required to determine the fair value of Legacy Korro’s common stock at the time of each grant of a stock-based award. Legacy Korro estimated the grant date fair value of its common stock using an appropriate valuation methodology, in accordance with the framework of the American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation Each valuation methodology includes estimates and assumptions that required the Legacy Korro’s judgment. These estimates and assumptions include a number of objective and subjective factors, including external market conditions, guideline public company information, the prices at which Legacy Korro sold convertible preferred stock to third parties in arms’ length transactions, the rights and preferences of securities senior to Legacy Korro’s common stock at the time and the likelihood of achieving a liquidity event such as an initial public offering or sale. Significant changes to the assumptions used in the valuations could result in different fair values of stock options and restricted stock at each valuation date, as applicable. In addition to the grant date fair value of the Company’s common stock, the Black-Scholes option pricing model requires the input of certain subjective assumptions, including (i) the calculation of expected term of the stock-based payment, (ii) the risk-free interest rate, (iii) the expected stock price volatility and (iv) the expected dividend yield. The Company uses the simplified method as proscribed by SEC Staff Accounting Bulletin No. 107 to calculate the expected term for stock options granted to employees as the Company does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. The Company determines the risk-free interest rate based on a treasury instrument whose term is consistent with the expected term of the stock options. Due to the lack of Company-specific historical and implied volatility data, the Company bases its estimates of expected volatility on the historical volatility of a group of publicly-traded companies with similar characteristics to itself, including stage of product development and therapeutic focus within the life sciences industry. The expected volatility has been determined using a weighted-average of the historical volatility measures of this group of companies. The Company expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. Historical volatility is calculated over a period of time commensurate with the expected term of the stock-based payment. The Company uses an assumed dividend yield of zero as the Company has never paid dividends on its common stock, nor does it expect to pay dividends on its common stock in the foreseeable future. The Company accounts for forfeitures of all stock-based payments when such forfeitures occur.
Income Taxes		Income Taxes Income taxes are recorded in accordance with ASC Topic 740, Income Taxes The Company accounts for uncertain tax positions using a more-likely-than-not Interest and penalty charges, if any, related to income taxes would be classified as a component of the “Provision for income taxes” in the consolidated statements of operations and comprehensive loss.
Net Loss per Share		Net Loss per Share Net loss per share attributable to common stockholders is calculated using the two-class Diluted net loss per share is computed using the more dilutive of (a) the two-class if-converted
Concentration of Credit Risk and Off-Balance Sheet Risk		Concentration of Credit Risk and Off-Balance Financial instruments that potentially expose the Company to concentrations of credit risk primarily consist of cash, cash equivalents and investments. Cash balances are deposited with federally-insured financial institutions in the United States and may, at times, exceed federally-insured limits. The Company maintains its cash, cash equivalents and investments with high-quality financial institutions and, consequently, the Company believes that such funds are subject to minimal credit risk. The Company’s cash equivalents are comprised of money market funds that are invested in U.S. Treasury and government agency obligations. The Company’s investments are comprised of commercial paper and government securities. Credit risk in these securities is reduced as a result of the Company’s investment policy to limit the amount invested in any single issuer and to only invest in securities of a high credit quality. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and to process its product candidates for its development programs. These programs could be adversely affected by a significant interruption in the manufacturing process or supply chain. The Company has no significant off-balance
Emerging growth company status		Emerging growth company status The Company qualifies as an “emerging growth company” (“EGC”), as defined in the Jumpstart Our Business Startups Act (“JOBS Act”), and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not EGCs. The Company may take advantage of these exemptions until it is no longer an EGC under Section 107 of the JOBS Act, which provides that an EGC can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to avail itself of the extended transition period and, therefore, while the Company is an EGC it will not be subject to new or revised accounting standards the same time that they become applicable to other public companies that are not EGCs, unless it chooses to early adopt a new or revised accounting standard. As a result of this election, the consolidated financial statements may not be comparable to companies that comply with public company FASB standards’ effective dates.
Recently Adopted Accounting Pronouncements		Recently Adopted Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments 2018-19, 2019-04, 2019-05, 2019-10, 2019-11, 2020-03, 2022-02 2016-13”). 2016-13
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 and other segment items and identifying the Chief Operating Decision Maker interim	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 FASB issued Accounting Standards Update (“ASU”) 2023-07, Segment Reporting (Topic 280), Improvements to Reportable Segment Disclosures

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Schedule of Financial Assets 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 Significant Significant 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 Significant Significant 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	Assets measured at fair value on a recurring basis as of December 31, 2023 were as follows (in thousands): Total Quoted Prices Significant Significant Money market funds, included in cash and cash equivalents $ 158,706 $ 158,706 $ —  $ —  MS APA asset $ 1,448 $ —  $ —  $ 1,448 Total $ 160,154 $ 158,706 $ —  $ 1,448 Assets measured at fair value on a recurring basis as of December 31, 2022 were as follows (in thousands): Total Quoted Prices Significant Significant Money market funds, included in cash and cash equivalents $ 14,904 $ 14,904 $ —  $ —  Short-term investments: Commercial paper $ 14,935 —  14,935 —  Government securities $ 3,980 —  3,980 —  Total $ 33,819 $ 14,904 $ 18,915 $ —

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2023
Investments [Abstract]
Schedule of Fair Value of Cash Equivalents and Short-Term Investments	Short-term investments as of December 31, 2022 were comprised as follows (in thousands): Amortized Unrealized Unrealized Fair Commercial paper $ 14,935 $ —  $ —  $ 14,935 Government securities 3,985 —  (5 ) 3,980 Total $ 18,920 $ —  $ (5 ) $ 18,915

Restricted Cash (Tables)

Restricted Cash (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Cash and Cash Equivalents [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	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, December 31, Cash and cash equivalents $ 138,804 $ 166,150 Restricted cash 6,952 6,969 Cash, cash equivalents and restricted cash $ 145,756 $ 173,119	The following table provides a reconciliation of cash, cash equivalents and restricted cash as of December 31, 2023 and 2022 that sums to the total of the same amounts shown in the consolidated statements of cash flows (in thousands): December 31, 2023 2022 Cash and cash equivalents $ 166,150 $ 36,333 Restricted cash 6,969 5,144 Cash, cash equivalents and restricted cash $ 173,119 $ 41,477

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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, December 31, 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	Property and equipment, net, as of December 31, 2023 and 2022 was comprised as follows (in thousands): December 31, Estimated Useful Life (in Years) 2023 2022 Laboratory equipment 5 $ 11,187 $ 8,441 Furniture and office equipment 4 481 477 Computer equipment 3 241 213 Leasehold improvements Shorter of useful life or remaining lease term 3,356 2,941 Construction in progress 8,242 2,049 Total property and equipment, gross 23,507 14,121 Less: accumulated depreciation (7,842 ) (4,255 ) Total property and equipment, net $ 15,665 $ 9,866

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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, December 31, 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	Accrued expenses and other current liabilities as of December 31, 2023 and 2022 were comprised as follows (in thousands): December 31, 2023 2022 Employee compensation and benefits $ 7,896 $ 2,624 External research and development services 1,255 274 Other operating expenses 447 207 CVR liability, current 395 —  Professional fees 219 70 Total accrued expenses and other current liabilities $ 10,212 $ 3,175

Common Stock (Tables)

Common Stock (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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, December 31, 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	As of December 31, 2023 and 2022, the Company had reserved for future issuance the following number of shares of common stock: December 31, 2023 2022 Conversion of outstanding Legacy Korro Series Seed Preferred Stock —  684,739 Conversion of outstanding Legacy Korro Series A Preferred Stock —  2,029,666 Conversion of outstanding Legacy Korro Series B-1 —  1,104,178 Conversion of outstanding Legacy Korro Series B-2 —  223,417 Future issuances of Legacy Korro Series B-2 —  813,239 Vesting of restricted common stock —  2,902 Exercises of outstanding stock options 1,323,151 461,128 Exercise of outstanding warrant 8,049 8,049 Future issuances under 2019 Stock Incentive Plan —  159,890 Future issuances under 2023 Stock Incentive Plan 255,694 —  Future issuances under 2023 ESPP Plan 88,502 —  Total reserved for future issuance 1,675,396 5,487,208

Preferred Stock (Tables)

Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2023
Class of Stock Disclosures [Abstract]
Schedule of Preferred Stock authorized, issued and outstanding Table	As of December 31, 2022, Legacy Korro Preferred Stock authorized, issued and outstanding consisted of the following (in thousands, except share amounts): December 31, 2022 Preferred Preferred Carrying Liquidation Common Legacy Korro Series Seed Preferred Stock 684,739 684,739 15,924 $ 16,115 684,739 Legacy Korro Series A Preferred Stock 2,029,666 2,029,666 77,736 91,500 2,029,666 Legacy Korro Series B-1 1,104,178 1,104,178 57,703 58,000 1,104,178 Legacy Korro Series B-2 1,036,656 223,417 12,500 12,500 223,417 Total 4,855,239 4,042,000 $ 163,863 $ 178,115 4,042,000

Stock-based Compensation (Table

Stock-based Compensation (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 2024 2023 Research and development $ 278 $ 90 General and administrative 587 259 Total stock-based compensation expense $ 865 $ 349	Total stock-based compensation expense recognized in the consolidated statements of operations and comprehensive loss for the years ended December 31, 2023 and 2022 was as follows (in thousands): Year Ended December 31, 2023 2022 Research and development $ 759 $ 243 General and administrative 1,622 897 Total stock-based compensation expense $ 2,381 $ 1,140
Summary of Restricted Common Stock Activity		The following table summarizes Legacy Korro restricted common stock activity during the year ended December 31, 2023: Shares Weighted- Unvested as of December 31, 2022 2,902 $ 0.59 Granted —  $ —  Vested 2,902 $ 0.23 Repurchased —  $ —  Unvested as of December 31, 2023 —  $ —
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 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 %	The fair value of stock options granted during the years ended December 31, 2023 and 2022 was calculated on the date of grant using the following weighted-average assumptions: Year Ended December 31, 2023 2022 Risk-free interest rate 4.3 % 2.5 % Expected dividend yield — % — % Expected term (in years) 6.0 6.0 Expected volatility 72.8 % 72.9 %
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- Weighted- Aggregate 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 following table summarizes changes in stock option activity during the year ended December 31, 2023 (in thousands, except per share amounts): Options Weighted- Weighted- Aggregate Outstanding at December 31, 2022 461,100 $ 14.95 8.0 $ 2,953 Assumed in reverse recapitalization 93,153 $ 121.91 Granted 835,839 $ 17.33 Exercised (39,395 ) $ 16.41 Forfeited (20,091 ) $ 20.07 Cancelled (7,455 ) $ 18.85 Outstanding as of December 31, 2023 1,323,151 $ 23.49 8.4 $ 36,310 Exercisable at December 31, 2023 368,036 $ 34.60 6.7 $ 8,457

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Schedule of Provision for Income Taxes	The provision for income taxes for the years ended December 31, 2023 and 2022 was comprised as follows (in thousands): Year Ended 2023 2022 Current taxes: Federal $ —  $ —  State 27 10 Total current taxes 27 10 Deferred taxes: Federal —  —  State —  —  Total deferred taxes —  —  Total provision for income taxes $ 27 $ 10
Schedule of Reconciliation of U.S. Federal Statutory Income Tax Rate	A reconciliation of the federal statutory income tax rate to the Company’s effective tax rate is as follows: December 31, 2023 2022 Income tax computed at federal statutory rate 21.0 % 21.0 % State taxes, net of federal benefit 7.1 % 6.1 % Tax credit carryforwards 3.6 % 6.2 % Permanent items (1.0 )% (0.2 )% Change in valuation allowance (30.8 )% (32.7 )% Other 0.1 % (0.4 )% Effective tax rate — % — %
Schedule of Deferred Tax Assets and Liabilities	The principal components of the Company’s deferred tax assets and liabilities as of December 31, 2023 and 2022 were comprised as follows (in thousands): December 31, 2023 2022 Deferred tax assets: Net operating loss carryforwards $ 80,385 $ 19,580 Tax credit carryforwards 22,474 5,477 Capitalized research and development 34,395 10,279 Stock-based compensation 4,494 319 Amortization 1,138 —  Operating lease liability 9,052 855 Accrued expenses and other temporary differences 1,543 707 Total deferred tax assets 153,481 37,217 Less: valuation allowance (145,766 ) (36,094 ) Net deferred tax assets 7,715 1,123 Deferred tax liabilities: Operating right-of-use (7,401 ) (552 ) Depreciation (314 ) (571 ) Total deferred tax liabilities (7,715 ) (1,123 ) Net deferred taxes $ —  $ —

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 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	Future minimum lease payments for all three leases, net of $10.8 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 December 31, 2023 were as follows (in thousands): As of December 31, 2023 2024 (7,227 ) 2025 6,247 2026 7,341 2027 7,557 Thereafter 52,498 Total Future Minimum Leases Payments 66,416 Less: Interest 33,209 Present Value of Operating Lease Liabilities 33,207
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 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	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): Year Ended December 31, 2023 2022 Operating lease costs $ 5,948 $ 1,675 Variable Lease Costs 1,211 732 Short-term Lease Costs 921 626 Total Lease Costs $ 8,080 $ 3,033

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
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 2024 2023 Legacy Korro Series Seed Preferred Stock —  684,739 Legacy Korro Series A Preferred Stock —  2,029,666 Legacy Korro Series B-1 —  1,104,178 Legacy Korro Series B-2 —  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	The following common stock equivalents have been excluded from the calculation of diluted net loss per share because their effect would be anti-dilutive: Year Ended December 31, 2023 2022 Legacy Korro Series Seed Preferred Stock —  684,739 Legacy Korro Series A Preferred Stock —  2,029,666 Legacy Korro Series B-1 —  1,104,178 Legacy Korro Series B-2 —  223,417 Unvested Legacy Korro restricted common stock —  2,902 Outstanding Legacy Korro stock options 1,323,151 461,100 Outstanding Legacy Korro warrant 8,049 8,049 Total 1,331,200 4,514,051
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 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 )	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): Years Ended December 31, 2023 2022 Numerator: Net loss $ (81,172 ) $ (58,032 ) Denominator: Weighted-average number of shares outstanding, basic and diluted 1,529,321 255,175 Net loss per share, basic and diluted $ (53.08 ) $ (227.42 )

The Company and Liquidity (Addi

The Company and Liquidity (Additional Information) (Details) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($) shares	Dec. 31, 2023 USD ($) $ / shares shares	Nov. 03, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) shares
Date of incorporation	Nov. 30, 2014	Nov. 30, 2014
Common stock, shares issued | shares	8,022,463	8,016,516		271,371
Exchange Ratio	0.049688	0.049688
Cash and cash equivalents	$ 138,804	$ 166,150	$ 36,600	$ 36,333
Accumulated deficit	(202,562)	(183,005)		$ (101,833)
MS APA Asset [Member]
Initial Consideration Amount	500	500
MS APA Asset [Member] | Minimum [Member]
Future Milestone Payments	700	700
MS APA Asset [Member] | Maximum [Member]
Future Milestone Payments	$ 17,500	$ 17,500
Common Stock [Member] | Legacy Korro [Member]
Ownership	91%	91%
Common Stock [Member] | Legacy Korro [Member] | Accredited Investors [Member]
Common stock, shares issued | shares			42,176,255
Common Stock, Value, Subscriptions			$ 117,300
Sale of Stock, Price Per Share | $ / shares			$ 2.78
Common Stock [Member] | Frequency [Member]
Common stock, shares issued | shares	8,623,645	8,623,645	7,848,776
Ownership	9%	9%
Preferred Stock [Member]
Exchange Ratio	0.049688	0.049688
Preferred Stock [Member] | Minimum [Member]
Sale of Stock, Price Per Share | $ / shares		$ 111.9
Preferred Stock [Member] | Legacy Korro [Member]
Exchange Ratio			0.049688

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Additional Information) (Details)	12 Months Ended
	Dec. 31, 2023 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in right-of-use asset	$ 0

Reverse Merger (Additional Info

Reverse Merger (Additional Information) (Details) - USD ($) $ in Thousands		12 Months Ended
	Nov. 03, 2023	Dec. 31, 2023	Mar. 31, 2024	Dec. 31, 2022
Business Combination and Asset Acquisition [Abstract]
Goodwill or intangible assets	$ 0
Cash and cash equivalents	36,600	$ 166,150	$ 138,804	$ 36,333
Prepaid expense and other assets	1,900
Accounts payable and accrued expenses	$ 5,400
Employee Benefits and Share-Based Compensation		300
Severance payment		2,500
Transaction costs		$ 12,247

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			$ 18,915
Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	$ 136,511	$ 160,154	33,819
Total financial liabilities	1,449	1,448
Fair Value, Inputs, Level 1 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	135,062	158,706	14,904
Total financial liabilities	0	0
Fair Value, Inputs, Level 2 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0	0	18,915
Total financial liabilities	0	0
Fair Value, Inputs, Level 3 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,449	1,448	0
Total financial liabilities	1,449	1,448
Money Market Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	135,062	158,706	14,904
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	135,062	158,706	14,904
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0	0	0
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0	0	0
MS APA Asset [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,449	1,448
MS APA Asset [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0	0
MS APA Asset [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	0	0
MS APA Asset [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value	1,449	1,448
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			14,935
Commercial Paper [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			14,935
Commercial Paper [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			0
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			14,935
Commercial Paper [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			0
CVR liability | Fair Value Measurements Recurring [Member]
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 Measurements Recurring [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 Measurements Recurring [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 Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial liabilities	$ 1,449	$ 1,448
Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			3,980
Government Securities [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			3,980
Government Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			0
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			3,980
Government Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value Measurements Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair market value			$ 0

Fair Value Measurements (Additi

Fair Value Measurements (Additional Information) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Tranfer of assets among fair value hierarchy levels	$ 0	$ 0	$ 0
Prepaid Expense and Other Assets, Current	3,305,000	3,015,000	1,232,000
Other non-current liabilities	$ 893,000	$ 1,053,000	$ 0
MS APA Asset [Member]
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Liabilities	Liabilities
Derivative Liability	$ 1,400,000	$ 1,400,000
Other current liabilities	600,000	400,000
Initial Consideration Amount	$ 500,000	$ 500,000
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Assets	Assets
Derivative Asset	$ 1,400,000	$ 1,400,000
Other Assets, Current	800,000	1,000,000
Prepaid Expense and Other Assets, Current	600,000	400,000
Other non-current liabilities	800,000	1,000,000
Maximum [Member] | MS APA Asset [Member]
Future Milestone Payments	17,500,000	17,500,000
Minimum [Member] | MS APA Asset [Member]
Future Milestone Payments	700,000	700,000
Fair Value, Inputs, Level 3 [Member]
Change in fair value of CVR liability		80,000
Fair Value, Inputs, Level 3 [Member] | MS APA Asset [Member]
Change in fair value of CVR liability	$ 0	$ 100,000

Investments - Schedule of Fair

Investments - Schedule of Fair Value of Cash Equivalents and Short-Term Investments (Details) $ in Thousands	Dec. 31, 2022 USD ($)
Cash and Cash Equivalents [Line Items]
Amortized Cost	$ 18,920
Unrealized Gains	0
Unrealized Losses	(5)
Fair Value	18,915
Government Securities [Member]
Cash and Cash Equivalents [Line Items]
Amortized Cost	3,985
Unrealized Gains	0
Unrealized Losses	(5)
Fair Value	3,980
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Amortized Cost	14,935
Unrealized Gains	0
Unrealized Losses	0
Fair Value	$ 14,935

Investments (Additional Informa

Investments (Additional Information) (Details) $ in Millions	Dec. 31, 2022 USD ($) Securities
Investments [Abstract]
Fair value of securities in unrealized loss position for less than twelve | $	$ 4
Number of position of securities in unrealized loss position for less than twelve | Securities	0

Restricted Cash - Schedule of R

Restricted Cash - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Nov. 03, 2023	Dec. 31, 2022
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 138,804	$ 166,150	$ 36,600	$ 36,333
Restricted Cash	6,952	6,969		5,144
Cash, cash equivalents and restricted cash	$ 145,756	$ 173,119		$ 41,477

Restricted Cash (Additional Inf

Restricted Cash (Additional Information) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Cash and Cash Equivalents [Line Items]
Restricted Cash, Current	$ 3,546	$ 3,563	$ 603
Restricted Cash, Noncurrent	3,400	3,400	$ 4,500
Current Assets [Member]
Cash and Cash Equivalents [Line Items]
Restricted Cash, Current	$ 3,600	$ 3,600

Property and Equipment, Net - S

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	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 29,076	$ 23,507	$ 14,121
Less: accumulated depreciation	(8,567)	(7,842)	(4,255)
Total property and equipment, net	20,509	15,665	9,866
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 11,586	$ 11,187	$ 8,441
Estimated Useful Life (in Years)	5 years	5 years	5 years
Furniture and office equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 481	$ 481	$ 477
Estimated Useful Life (in Years)	4 years	4 years	4 years
Computer equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 241	$ 241	$ 213
Estimated Useful Life (in Years)	3 years	3 years	3 years
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 3,356	$ 3,356	$ 2,941
Property, Plant and Equipment, Dispositions	Shorter of useful life or remaining lease term	Shorter of useful life or remaining lease term	Shorter of useful life or remaining lease term
Construction in progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 13,412	$ 8,242	$ 2,049

Property and Equipment, Net (Ad

Property and Equipment, Net (Additional Information) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Abstract]
Depreciation	$ 726	$ 809	$ 3,630	$ 2,511
Construction in progress	$ 13,400		$ 8,200	$ 2,000

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Employee compensation and benefits	$ 2,675	$ 7,896	$ 2,624
External research and development services	1,577	1,255	274
Other operating expenses	140	447	207
CVR Liability, Current	556	395	0
Professional fees	213	219	70
Total accrued expenses and other current liabilities	$ 5,161	$ 10,212	$ 3,175

Common Stock - Schedule of Numb

Common Stock - Schedule of Number of Shares Reserved for Future Issuance (Details) - shares	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Preferred stock, shares outstanding	0	0	0
Future issuance of share	8,022,463	8,016,516	271,371
Total reserved for future issuance	2,150,440	1,675,396	5,487,208
Exercises of outstanding Shares	8,022,463	8,016,516	268,468
2019 Stock Incentive Plan [Member]
Future issuance of share		0	159,890
Total reserved for future issuance		110,285
2023 Stock Incentive Plan [Member]
Future issuance of share	700,005	255,694	0
Employee Stock Purchase Plan [Member]
Future issuance of share	168,667	88,502	0
Outstanding Warrant [Member]
Exercises of outstanding Shares	8,049	8,049	8,049
Stock Options [Member]
Exercises of outstanding Shares	1,273,719	1,323,151	461,128
Restricted Common Stock [Member]
Future issuance of share		0	2,902
Series Seed Preferred Stock [Member]
Preferred stock, shares outstanding		0	684,739
Series A Preferred Stock [Member]
Preferred stock, shares outstanding		0	2,029,666
Series B-1 Preferred Stock [Member]
Preferred stock, shares outstanding		0	1,104,178
Series B-2 Preferred Stock [Member]
Preferred stock, shares outstanding		0	223,417
Future issuance of share		0	813,239

Common Stock (Additional Inform

Common Stock (Additional Information) (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Common stock, shares authorized	200,000,000	200,000,000	5,755,759
Common stock, par value	$ 0.001	$ 0.001	$ 0.001
Korros [Member]
Dividends Declared	$ 0	$ 0
Korros [Member] | Common Stock [Member]
Common stock, shares authorized	200,000,000	200,000,000
Common stock, par value	$ 0.001	$ 0.001

Preferred Stock (Additional Inf

Preferred Stock (Additional Information) (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) $ / shares shares	Mar. 31, 2024 $ / shares shares	Nov. 03, 2023	Dec. 31, 2022 $ / shares shares
Class of Stock [Line Items]
Preferred stock, shares issued	0	0		0
Preferred Stock, Shares Authorized	10,000,000	10,000,000		0
Preferred stock, par value | $ / shares	$ 0.001	$ 0.001		$ 0.001
Preferred Stock, Shares Outstanding	0	0		0
Preferred Stock, Convertible, Conversion Ratio	0.049688	0.049688
Preferred Stock [Member]
Class of Stock [Line Items]
Percentage of outstanding shares of Preferred Stock	66%	66%
Preferred Stock, Convertible, Conversion Ratio	0.049688	0.049688
Preferred Stock, Dividend Rate, Percentage	8%
Preferred Stock [Member] | Minimum [Member]
Class of Stock [Line Items]
Sale of Stock, Price Per Share | $ / shares	$ 111.9
Percentage of outstanding shares of Preferred Stock	66%
Proceeds from Issuance Initial Public Offering | $	$ 75
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Sale of Stock, Price Per Share | $ / shares	$ 45.08
Preferred Stock, Shares Outstanding	0			2,029,666
Preferred Stock, Convertible, Conversion Price | $ / shares	$ 45.08
Series B-1 Preferred Stock [Member]
Class of Stock [Line Items]
Sale of Stock, Price Per Share | $ / shares	52.53
Preferred Stock, Convertible, Conversion Price | $ / shares	52.53
Series B-2 Preferred Stock [Member]
Class of Stock [Line Items]
Sale of Stock, Price Per Share | $ / shares	55.95
Preferred Stock, Convertible, Conversion Price | $ / shares	55.95
Series Seed Preferred Stock [Member]
Class of Stock [Line Items]
Sale of Stock, Price Per Share | $ / shares	$ 20.13
Preferred Stock, Shares Outstanding	0			684,739
Preferred Stock, Convertible, Conversion Price | $ / shares	$ 20.13
Series Seed-3 Preferred Stock [Member]
Class of Stock [Line Items]
Sale of Stock, Price Per Share | $ / shares	26.16
Preferred Stock, Convertible, Conversion Price | $ / shares	$ 26.16
Legacy Korro [Member]
Class of Stock [Line Items]
Preferred stock, shares issued				4,042,000
Preferred Stock, Shares Authorized				4,855,239
Preferred Stock, Shares Outstanding				4,042,000
Legacy Korro [Member] | Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued				0
Preferred Stock, Shares Authorized				10,000,000
Preferred stock, par value | $ / shares				$ 0.001
Preferred Stock, Shares Outstanding				0
Preferred Stock, Convertible, Conversion Ratio			0.049688
Legacy Korro [Member] | Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued				2,029,666
Preferred Stock, Shares Authorized				2,029,666
Preferred Stock, Shares Outstanding				2,029,666
Legacy Korro [Member] | Series B-1 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued				1,104,178
Preferred Stock, Shares Authorized				1,104,178
Preferred Stock, Shares Outstanding				1,104,178
Legacy Korro [Member] | Series B-2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued				223,417
Preferred Stock, Shares Authorized				1,036,656
Preferred Stock, Shares Outstanding				223,417
Legacy Korro [Member] | Series Seed Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued				684,739
Preferred Stock, Shares Authorized				684,739
Preferred Stock, Shares Outstanding				684,739

Preferred Stock - Schedule Of P

Preferred Stock - Schedule Of Preferred Stock Authorized, Issued And Outstanding Table (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Class of Stock [Line Items]
Preferred stock, shares authorized	10,000,000	10,000,000	0
Preferred Stock Outstanding	0	0	0
Preferred Stock Issued	0	0	0
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at December 31, 2023 and 2022; no shares issued and outstanding at December 31, 2023 and 2022	$ 0	$ 0	$ 0
Series Seed Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock Outstanding		0	684,739
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock Outstanding		0	2,029,666
Legacy Korro [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized			4,855,239
Preferred Stock Outstanding			4,042,000
Preferred Stock Issued			4,042,000
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at December 31, 2023 and 2022; no shares issued and outstanding at December 31, 2023 and 2022			$ 163,863
Liquidation Value			$ 178,115
Common Stock Issuable Upon Conversion			4,042,000
Legacy Korro [Member] | Series Seed Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized			684,739
Preferred Stock Outstanding			684,739
Preferred Stock Issued			684,739
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at December 31, 2023 and 2022; no shares issued and outstanding at December 31, 2023 and 2022			$ 15,924
Liquidation Value			$ 16,115
Common Stock Issuable Upon Conversion			684,739
Legacy Korro [Member] | Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized			2,029,666
Preferred Stock Outstanding			2,029,666
Preferred Stock Issued			2,029,666
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at December 31, 2023 and 2022; no shares issued and outstanding at December 31, 2023 and 2022			$ 77,736
Liquidation Value			$ 91,500
Common Stock Issuable Upon Conversion			2,029,666
Legacy Korro [Member] | Series B-1 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized			1,104,178
Preferred Stock Outstanding			1,104,178
Preferred Stock Issued			1,104,178
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at December 31, 2023 and 2022; no shares issued and outstanding at December 31, 2023 and 2022			$ 57,703
Liquidation Value			$ 58,000
Common Stock Issuable Upon Conversion			1,104,178
Legacy Korro [Member] | Series B-2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized			1,036,656
Preferred Stock Outstanding			223,417
Preferred Stock Issued			223,417
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at December 31, 2023 and 2022; no shares issued and outstanding at December 31, 2023 and 2022			$ 12,500
Liquidation Value			$ 12,500
Common Stock Issuable Upon Conversion			223,417

Stock-based Compensation (Addit

Stock-based Compensation (Additional Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Nov. 30, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, reserved for future issuance	2,150,440		1,675,396	5,487,208
Share-Based Compensation Arrangement by Share-Based Payment Award, Description			In addition, the number of shares of common stock available for issuance under the 2023 Plan will be automatically increased on the first day of each calendar year during the ten-year term of the 2023 Plan, beginning with January 1, 2024 and ending with January 1, 2033, by the amount equal to 5% of the outstanding number of shares of the Company’s common stock on December 31 of the preceding calendar year or such lesser amount as determined by the Company’s board of directors.
Number of Outstanding Option	1,273,719		1,323,151	461,100
Weighted-average grant date fair value of stock options granted	$ 21.43	$ 23.49	$ 23.49	$ 14.95
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period, Intrinsic Value	$ 0.1	$ 0	$ 0.3	$ 0.1
Total unrecognized stock-based compensation expense relating to unvested stock options	$ 9.6		$ 10.1
Unrecognized unvested stock options, weighted-average period	3 years		3 years 1 month 6 days
Options vesting period			4 years
Options expiration period			10 years
Restricted Stock Units (RSUs) [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Fair value of restricted common stock	$ 0	$ 0.1	$ 0.1	$ 0.5
2019 Stock Incentive Plan Member
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, reserved for future issuance			600,977	683,156
2019 Stock Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, reserved for future issuance			110,285
Common stock authorized under plan		747,752
2019 Incentive Award Plan [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock authorized under plan			154,032
2023 Stock Option and Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, reserved for future issuance			889,922		885,028
Common stock, shares available for future grants			255,694
2019 Stock Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of Outstanding Option			51,491
2014 Stock Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, reserved for future issuance			424,853
Common stock, shares available for future grants			0
Number of Outstanding Option			36,455
Legacy Korro [Member] | 2019 Stock Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, shares available for future grants			0	159,890

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation	$ 865	$ 349	$ 2,381	$ 1,140
Research and Development Expense [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation	278	90	759	243
General and Administrative Expense [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock-based compensation	$ 587	$ 259	$ 1,622	$ 897

Stock-based Compensation - Su_2

Stock-based Compensation - Summary of Restricted Common Stock Activity (Details) - Restricted Stock Units (RSUs) [Member]	12 Months Ended
	Dec. 31, 2023 $ / shares shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Unvested as of December 31, 2022 | shares	2,902
Granted | shares	0
Vested | shares	2,902
Repurchased | shares	0
Unvested as of December 31, 2023 | shares	0
Weighted average fair value, Unvested as of December 31, 2022 | $ / shares	$ 0.59
Weighted average fair value, Granted | $ / shares	0
Weighted average fair value, Vested | $ / shares	0.23
Weighted average fair value, Repurchased | $ / shares	0
Weighted average fair value, Unvested as of December 31, 2023 | $ / shares	$ 0

Stock-based Compensation - Su_3

Stock-based Compensation - Summary of Stock Option Weighted-Average Assumptions (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]
Risk-free interest rate	4.10%	3.70%	4.30%	2.50%
Expected dividend yield	0%	0%	0%	0%
Expected term (in years)	6 years	6 years	6 years	6 years
Expected volatility	74.10%	69.70%	72.80%	72.90%

Stock-based Compensation - Su_4

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	Dec. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares Options , Beginning balance	1,323,151	461,100
Number of Share Option, Reverse Recapitalization		93,153
Number of shares Options , Granted	24,926	835,839
Number of shares Options, Exercised	(5,947)	(39,395)
Number of shares Options, Forfeited	(32,174)	(20,091)
Number of shares Options, Cancelled	(36,237)	(7,455)
Number of shares Options, Ending balance	1,273,719	1,323,151	461,100
Exercisable at March 31, 2024	434,915	368,036
Weighted average exercise price, Beginning balance	$ 23.49	$ 14.95
Weighted- Average Exercise Price Reverse Recapitalization		121.91
Weighted average exercise price, Granted	46.65	17.33
Weighted average exercise price, Exercised	23.38	16.41
Weighted average exercise price, Fofeited	18.09	20.07
Weighted- Average Exercise Price Cancelled	117.15	18.85
Weighted average exercise price, Ending balance	21.42	23.49	$ 14.95
Weighted average exercise price, Options Exercisable, Ending balance	$ 27.76	$ 34.6
Weighted average remaining contractual term (in years), Outstanding	8 years 6 months	8 years 4 months 24 days	8 years
Weighted average remaining contractual term (in years), Options exercisable as of December 31, 2022	7 years	6 years 8 months 12 days
Aggregate intrinsic value, Outstanding, Beginning balance	$ 36,310	$ 2,953
Aggregate intrinsic value, Outstanding, Ending balance	85,695	36,310	$ 2,953
Aggregate intrinsic value, Options exercisable as of Exercisable	$ 25,584	$ 8,457

Income taxes (Additional Inform

Income taxes (Additional Information) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Operating Loss Carryforwards [Line Items]
Net Operating Loss Carryforwards, Limitations on Use	Under current law, the Company’s federal NOLs generated in taxable years ending after December 31, 2017, may be carried forward indefinitely, but the deductibility of such federal NOLs is limited to 80% of the Company’s taxable income annually for tax years beginning after December 31, 2018. Federal NOLs generated in taxable years ending on or prior to December 31, 2017, however, have a 20-year carryforward period, but are not subject to the 80% limitation.
Net Operating Loss Carryforwards Expiration without Limitation	$ 280,100
Tax Credit Carryforward, Amount	22,474	$ 5,477
Valuation Allowance	145,766	36,094
Change in Valuation Allowance Amount	$ 109,700
Explanation of Change in Valuation Allowance	the additional operating loss generated by the Company and the combination of deferred tax assets as a result of the Merger.
Unrecognized Tax Benefits	$ 0	$ 0
Penalties Charges and Interest Expense	$ 0
Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Period of Cumulative Change in Ownership	3 years
Maximum [Member]
Operating Loss Carryforwards [Line Items]
Net Operating Loss Carryforwards, Expiration Year	2037
Minimum [Member]
Operating Loss Carryforwards [Line Items]
Net Operating Loss Carryforwards, Expiration Year	2036
Minimum [Member] | Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Cumulative Change In Ownership Percentage	50%
State [Member]
Operating Loss Carryforwards [Line Items]
Net Operating Loss Carryforwards	$ 266,300
State [Member] | Research and Development Tax Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforward, Amount	$ 8,700
State [Member] | Maximum [Member]
Operating Loss Carryforwards [Line Items]
Net Operating Loss Carryforwards, Expiration Year	2043
State [Member] | Maximum [Member] | Research and Development Tax Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforward, Expiration Year	2038
State [Member] | Minimum [Member]
Operating Loss Carryforwards [Line Items]
Net Operating Loss Carryforwards, Expiration Year	2035
State [Member] | Minimum [Member] | Research and Development Tax Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforward, Expiration Year	2032
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net Operating Loss Carryforwards	$ 302,500
Net Operating Loss Carryforwards Expiration with Limitation	22,400
Federal [Member] | Research and Development Tax Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforward, Amount	$ 15,600
Federal [Member] | Maximum [Member] | Research and Development Tax Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforward, Expiration Year	2043
Federal [Member] | Minimum [Member] | Research and Development Tax Credit Carryforward [Member]
Operating Loss Carryforwards [Line Items]
Tax Credit Carryforward, Expiration Year	2036

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Current taxes:
Federal			$ 0	$ 0
State			27	10
Total current taxes			27	10
Deferred taxes:
Federal			0	0
State			0	0
Total deferred taxes			0	0
Total provision for income taxes	$ (17)	$ 0	$ 27	$ 10

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of U.S. Federal Statutory Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory rate	21%	21%
State taxes, net of federal benefit	7.10%	6.10%
Tax credit carryforwards	3.60%	6.20%
Permanent items	(1.00%)	(0.20%)
Change in valuation allowance	(30.80%)	(32.70%)
Other	0.10%	(0.40%)
Effective tax rate	0%	0%

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Net operating loss carryforwards	$ 80,385	$ 19,580
Tax credit carryforwards	22,474	5,477
Capitalized research and development	34,395	10,279
Stock-based compensation	4,494	319
Amortization	1,138	0
Operating lease liability	9,052	855
Accrued expenses and other temporary differences	1,543	707
Total deferred tax assets	153,481	37,217
Less: valuation allowance	(145,766)	(36,094)
Net deferred tax assets	7,715	1,123
Deferred tax liabilities:
Operating right-of-use asset	(7,401)	(552)
Depreciation	(314)	(571)
Total deferred tax liabilities	(7,715)	(1,123)
Net deferred taxes	$ 0	$ 0

Genevant Agreement (Additional

Genevant Agreement (Additional Information) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense, Total	$ 13,572	$ 14,694	$ 57,250	$ 42,201
Reimbursements with in Research and Development Expense	900	$ 2,500	1,200
Genevant Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and Development Expense, Total	2,500		2,500
Revenue recognized commercialization milestone payments to receive	57,000		57,000
Genevant Agreement [Member] | Maximum [Member] | Phase 3 Clinical Trial
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	13,500		13,500
Genevant Agreement [Member] | Maximum [Member] | Phase2b Clinical Trial
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 27,000		$ 27,000

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Future Minimum Rental Payments (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Lessee, Lease, Description [Line Items]
2024	$ (5,464)
2025	6,247	$ 7,227
2026	7,341	6,247
2027	7,557	7,341
2027		7,557
Thereafter	52,498	52,498
Total Future Minimum Leases Payments	68,179	66,416
amount of lease payments representing interest	(32,263)	(33,209)
Present Value of Operating Lease Liabilities	$ 35,916	$ 33,207

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
Operating lease costs	$ 2,150	$ 481	$ 5,948	$ 1,675
Variable Lease Costs	359	258	1,211	732
Short-term Lease Costs	409	181	921	626
Total Lease Costs	$ 2,918	$ 920	$ 8,080	$ 3,033

Commitments and Contingencies_3

Commitments and Contingencies (Additional Information) (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($) ft²	Dec. 31, 2023 USD ($) ft²	Sep. 30, 2023 USD ($)	May 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets	$ 25,947	$ 27,150			$ 2,024
Operating lease liability	35,916	33,207
Lease rental payments	$ 68,179	$ 66,416
Weighted average remaining lease term	9 years 10 months 24 days	10 years
Weighted average remaining lease discount rate	11.20%	11.20%
Loss contingency accrual	$ 0	$ 0
OKS Facility [Member]
Lessee, Lease, Description [Line Items]
Lease area of land | ft²	22,561	22,561
Lease expiration date	Oct. 20, 2023	Oct. 20, 2023
Cummings Park Sublease [Member]
Lessee, Lease, Description [Line Items]
Lease area of land | ft²	18,148	18,148
Lease expiration date	Jul. 31, 2024
60 First Street Lease [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Remaining Lease Term	10 years	10 years
Lease area of land | ft²	50,453	50,453
Lease expiration date		Jul. 31, 2024
Operating lease right-of-use assets				$ 26,800
Operating lease liability				26,800
Lease rental payments				62,100
Tenant improvement allowance	$ 13,100	$ 13,100		$ 13,100
Tenant improvement allowance utilized	4,800	$ 2,300
Future minimum lease payments receivable	$ 8,300		$ 10,800

401(k) Savings Plan (Additional

401(k) Savings Plan (Additional Information) (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
	Apr. 01, 2022	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Defined Contribution Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	3%
Defined Contribution Plan, Cost		$ 0.2	$ 0.2	$ 0.5	$ 0.2
Maximum [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100%

Related Party Transactions (Add

Related Party Transactions (Additional Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Related Party Transaction [Line Items]
Incurred expenses	$ 0
Affiliate amount related to consulting services	$ 0	$ 0
Maximum [Member]
Related Party Transaction [Line Items]
Incurred expenses		$ 0.1

Net Loss per Share - Computatio

Net Loss per Share - Computation of Diluted Net Loss per Share Attributable to Common Stockholders (Details) - Legacy Korro [Member] - shares	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Antidilutive securities excluded from computation of earnings per share	1,281,768	5,417,353	1,331,200	4,514,051
Outstanding Warrant [Member]
Antidilutive securities excluded from computation of earnings per share	8,049	8,049	8,049	8,049
Unvested Restricted Stock Units [Member]
Antidilutive securities excluded from computation of earnings per share	0	1,465	0	2,902
Outstanding Stock Options [Member]
Antidilutive securities excluded from computation of earnings per share	1,273,719	552,600	1,323,151	461,100
Series Seed Preferred Stock [Member]
Antidilutive securities excluded from computation of earnings per share	0	684,739	0	684,739
Series A Preferred Stock [Member]
Antidilutive securities excluded from computation of earnings per share	0	2,029,666	0	2,029,666
Series B-1 Preferred Stock [Member]
Antidilutive securities excluded from computation of earnings per share	0	1,104,178	0	1,104,178
Series B-2 Preferred Stock [Member]
Antidilutive securities excluded from computation of earnings per share	0	1,036,656	0	223,417

Net Loss per Share - Schedule o

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		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Numerator:
Net loss	$ (19,557)	$ (19,605)	$ (81,172)	$ (58,032)
Denominator:
Weighted-average shares used in computing net loss per share, basic	8,019,626	271,522	1,529,321	255,175
Weighted-average shares used in computing net loss per share, diluted	8,019,626	271,522	1,529,321	255,175
Net loss per share, basic	$ (2.44)	$ (72.2)	$ (53.08)	$ (227.42)
Net loss per share, diluted	$ (2.44)	$ (72.2)	$ (53.08)	$ (227.42)

Subsequent Events (Additional I

Subsequent Events (Additional Information) (Details) - USD ($) $ in Millions	Apr. 17, 2024	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Subsequent Event [Line Items]
Common Stock, Shares, Issued		8,022,463	8,016,516	271,371
Subscription agreement [Member] | Private Placement [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Proceeds from Issuance of Private Placement	$ 70
Common Stock, Shares, Issued	1,249,283