Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 328

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 328
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 policy is to account for
forfeitures when they occur. Stock-based compensation expense is classified in the accompanying consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in
which the award recipients service payments are classified.

Restricted common stock awards are subject to repurchase rights until such awards meet
all vesting conditions. Accordingly, the Company has recorded the proceeds from the issuance of restricted stock awards as a liability in the consolidated balance sheets. The restricted stock liability is reclassified into stockholders’ equity
as the restricted stock vests.

The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing
model. The Board of Directors of the Company (the “Board”) determines the fair value of the Company’s common stock, par value $0.001 per share (“common stock”), taking into consideration its most recently available
third-party valuations of common stock as well as additional factors which may have changed since the date of the most recent contemporaneous valuation through grant date. Due to the lack of company-specific historical and implied volatility
information, the Company estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until it has adequate historical data regarding the volatility of its own
traded stock price. The Company uses the simplified method as prescribed by the

F-12

Securities and Exchange Commission’s Staff Accounting Bulletin No. 107, Share-Based Payment, to calculate the expected term for options granted to employees and non-employees as
the Company does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of
grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is zero because the Company has never paid cash dividends on common shares and does not expect to pay any cash dividends in the
foreseeable future.

Income Taxes

The
Company’s provision for income taxes, deferred tax assets and liabilities, and reserves for unrecognized tax benefits reflect our best assessment of estimated future taxes to be paid. Significant judgments and estimates based on interpretations
of existing tax laws or regulations in the United States are required in determining our provision for income taxes. Changes in tax laws, statutory tax rates, and estimates of our future taxable income could impact the deferred tax assets and
liabilities