Alumis and ACELYRIN Announce Amended Merger Agreement ACELYRIN stockholders to receive increased ownership in the combined company through revised exchange ratio; Alumis and ACELYRIN stockholders to now own approximately

Alumis and ACELYRIN Announce Amended Merger

ACELYRIN stockholders to receive increased ownership

in the combined company through revised exchange ratio; Alumis and ACELYRIN stockholders to now own approximately 52% and 48%, respectively,

of the combined company on a fully diluted basis

Merger maximizes the potential value for ACELYRIN

stockholders and creates a stronger combined company, best-positioned to realize long-term value of multiple late-stage assets

ACELYRIN files investor presentation highlighting

benefits of proposed merger and comprehensive Board process

Special Meeting of Stockholders for both companies

to be held May 13, 2025

SOUTH SAN FRANCISCO, Calif. and LOS ANGELES, Calif. - April 21,

2025 - Alumis Inc. (Nasdaq: ALMS), a clinical-stage biopharmaceutical company developing therapies using a precision approach

to optimize clinical outcomes and significantly improve the lives of patients with immune-mediated diseases, and ACELYRIN, INC. (Nasdaq:

SLRN), a late-stage clinical biopharma company focused on accelerating the development and delivery of transformative medicines in immunology,

today announced an amendment to the existing terms of their previously announced merger agreement.

Under the terms of the amended agreement, ACELYRIN stockholders will

now receive 0.4814 shares of Alumis common stock for each share of ACELYRIN common stock owned, representing a meaningful increase in

the ownership percentage of the combined company over the original definitive merger agreement. With the amended exchange ratio, Alumis

stockholders will own approximately 52% of the combined company and ACELYRIN stockholders will own approximately 48% on a fully diluted

Martin Babler, President, Chief Executive Officer and Chairman of Alumis,

said, "In recognition of the current market conditions and evolving investor expectations for a successful combination, we have

revised the terms of our agreement with ACELYRIN, enabling enhanced value creation opportunities for our respective stockholders. This

was carefully considered by our Board of Directors and we continue to firmly believe in the merits of the transaction. This merger provides

Alumis the best opportunity to significantly enhance our financial flexibility and runway to advance an expanded late-stage pipeline with

multiple near-term development milestones and build commercial capabilities to maximize the value of our portfolio for patients and stockholders.

We will continue to work closely with ACELYRIN to successfully complete the transaction and deliver on its significant benefits."

Bruce Cozadd, Chair of the ACELYRIN Board of Directors and member of

the Board Transaction Committee, said, "Since announcing the merger, we have had extensive conversations with our stockholders who

have expressed an understanding of the strategic rationale for this transaction, while also sharing their perspectives on the value provided

to ACELYRIN stockholders. This amended agreement reflects this dialogue with stockholders and meaningfully builds upon the previously

announced agreement, which was the result of a rigorous, objective, and competitive process facilitated by the ACELYRIN Board. Because

of the Board's continued efforts, our stockholders now stand to benefit from a greater interest in Alumis' long-term upside

potential. We continue to believe that this combination is the most value-maximizing path forward for ACELYRIN stockholders and that Alumis

is the right partner to optimize development of lonigutamab."

ACELYRIN also filed today an investor presentation with the U.S. Securities

and Exchange Commission ("SEC") highlighting additional details and benefits of the amended merger agreement, including:

The presentation is available on ACLEYRIN's investor relations

website at https://investors.acelyrin.com/.

The amended merger agreement was unanimously recommended and approved

by the disinterested directors of each company's Board. As previously announced, Stockholders representing approximately 62% of

Alumis voting common stock and approximately 24% of ACELYRIN common stock have entered into voting agreements in support of the transaction.

Alumis and ACELYRIN intend to file supplemental proxy materials with

the Securities and Exchange Commission promptly. The companies continue to expect to close the transaction during the second quarter of

2025, subject to the approval by both companies' stockholders and satisfaction of other customary closing conditions.

As previously disclosed, Alumis and ACELYRIN will hold its respective

Special Meeting of Stockholders on May 13, 2025, and stockholders of record as of the close of business on April 1, 2025, are

entitled to vote at the Special Meetings.

Morgan Stanley & Co. LLC is serving

as financial advisor to Alumis, and Cooley LLP is serving as its legal counsel. Guggenheim Securities, LLC is serving as financial

advisor to ACELYRIN and Fenwick & West LLP and Paul Hastings LLP are serving as legal counsel.

Alumis is a clinical-stage biopharmaceutical

company developing oral therapies using a precision approach to optimize clinical outcomes and significantly improve the lives of patients

with immune-mediated diseases. Leveraging its proprietary precision data analytics platform, Alumis is building a pipeline of molecules

with the potential to address a broad range of immune-mediated diseases as monotherapy or combination therapies. Alumis' most advanced

product candidate, ESK-001, is an oral, highly selective, small molecule, allosteric inhibitor of tyrosine kinase 2 that is currently

being evaluated for the treatment of patients with moderate-to-severe plaque psoriasis and systemic lupus erythematosus. Alumis is also

developing A-005, a CNS-penetrant, allosteric TYK2 inhibitor for the treatment of neuroinflammatory and neurodegenerative diseases. Beyond

TYK2, Alumis' proprietary precision data analytics platform and drug discovery expertise have led to the identification of additional

preclinical programs that exemplify its precision approach. Incubated by Foresite Labs and led by a team of industry veterans experienced

in small-molecule compound drug development for immune-mediated diseases, Alumis is pioneering a precision approach to drug development

to potentially produce the next generation of treatment to address immune dysfunction.

ACELYRIN, INC. (Nasdaq: SLRN) is focused on providing patients

life-changing new treatment options by identifying, acquiring, and accelerating the development and commercialization of transformative

medicines. ACELYRIN's lead program, lonigutamab, is a subcutaneously delivered monoclonal antibody targeting IGF-1R being investigated

for the treatment of thyroid eye disease.

Forward-Looking Statements

This communication contains forward-looking statements within the meaning

of federal securities laws, including the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995.

Such statements are based upon current plans, estimates and expectations of management of Alumis Inc. ("Alumis") and ACELYRIN, Inc.

("ACELYRIN") in light of historical results and trends, current conditions and potential future developments, and are subject

to various risks and uncertainties that could cause actual results to differ materially from such statements. The inclusion of forward-looking

statements should not be regarded as a representation that such plans, estimates and expectations will be achieved. Words such as "anticipate,"

"expect," "project," "intend," "believe," "may," "will," "should,"

"plan," "could," "continue," "target," "contemplate," "estimate,"

"forecast," "guidance," "predict," "possible," "potential," "pursue,"

"likely," and words and terms of similar substance used in connection with any discussion of future plans, actions or events

identify forward-looking statements. All statements, other than statements of historical facts, including express or implied statements

regarding the proposed transaction; the conversion of equity interests contemplated by the agreement and plan of merger, dated as of February 6,

2025, as amended on April 20, 2025, by and among the parties (as amended, the "merger agreement"); the issuance of common

stock of Alumis contemplated by the merger agreement; the expected filing by Alumis with the Securities and Exchanges Commission (the

"SEC") of a registration statement on Form S-4 (the "registration statement") and a joint proxy statement/prospectus

of Alumis and ACELYRIN to be included therein (the "joint proxy statement/prospectus"); the expected timing of the closing

of the proposed transaction; the ability of the parties to complete the proposed transaction considering the various closing conditions;

the expected benefits of the proposed transaction; the sufficiency of the combined company's capital resources; the combined company's

cash runway, which is preliminary, unaudited and subject to change; the competitive ability and position of the combined company; the

clinical pipeline of the combined company; and any assumptions underlying any of the foregoing, are forward-looking statements.

Risks and uncertainties include, among other things, (i) the risk

that the proposed transaction may not be completed in a timely basis or at all, which may adversely affect Alumis' and ACELYRIN's

businesses and the price of their respective securities; (ii) the potential failure to receive, on a timely basis or otherwise, the

required approvals of the proposed transaction, including stockholder approvals by both Alumis' stockholders and ACELYRIN'S

stockholders, and the potential failure to satisfy the other conditions to the consummation of the transaction; (iii) the effect

of the announcement, pendency or completion of the proposed transaction on each of Alumis' or ACELYRIN's ability to attract,

motivate, retain and hire key personnel and maintain relationships with partners, suppliers and others with whom Alumis or ACELYRIN does

business, or on Alumis' or ACELYRIN's operating results and business generally; (iv) that the proposed transaction may

divert management's attention from each of Alumis' and ACELYRIN's ongoing business operations; (v) the risk of

any legal proceedings related to the proposed transaction or otherwise, or the impact of the proposed transaction thereupon, including

resulting expense or delay; (vi) that Alumis or ACELYRIN may be adversely affected by other economic, business and/or competitive

factors; (vii) the occurrence of any event, change or other circumstance that could give rise to the termination of the merger agreement

, including in circumstances which would require Alumis or ACELYRIN to pay a termination fee; (viii) the risk that restrictions during

the pendency of the proposed transaction may impact Alumis' or ACELYRIN's ability to pursue certain business opportunities