Company: SION
Filing Date: 2025-01-17
Form Type: S-1
Source: 0001193125-25-008474
Chunk: 298

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-01-17
Form: S-1
Chunk 298
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30% (or such lower rate specified by an applicable income tax treaty) of its effectively connected dividends, as adjusted for certain items. Amounts not treated as dividends for U.S. federal income tax purposes will constitute a return of capital and will first be applied against and reduce a holder’s tax basis in our common stock, but not below zero. Any excess will be treated as gain realized on the sale or other disposition of our common stock and will be treated as described under “ —Gain on Sale or Other Taxable Disposition of Our Common Stock” below. Gain on Sale or Other Taxable Disposition of Our Common Stock Subject to the discussion below regarding backup withholding and FATCA (as defined below), a non-U.S.holder generally will not be subject to U.S. federal income tax on any gain realized on the sale or other taxable disposition of our common stock, unless:

| • |     | the gain is effectively connected with the non-U.S. holder’s conduct of a                                                                                                                    
 trade or business in the U.S. and, if required by an applicable income tax treaty, is attributable to a permanent establishment or fixed base maintained by the non-U.S. holder in the U.S.; |

| • |     | the non-U.S. holder is a nonresident alien individual who is present in the U.S.                                                            
 for a period or periods aggregating 183 days or more during the taxable year of the disposition, and certain other requirements are met; or |

Gain described in the first bullet point above generally will be subject to U.S. federal income tax on a net income basis at the regular U.S. federal income tax rates in the same manner as if such holder were a resident of the U.S. A non-U.S.holder that is a foreign corporation also may be subject to an additional branch profits tax equal to 30% (or such lower rate specified by an applicable income tax treaty) of its effectively connected earnings and profits for the taxable year, as adjusted for certain items. Gain described in the second bullet point above will be subject to U.S. federal income tax at a flat 30% rate (or such lower rate specified by an applicable income tax treaty), but may be offset by certain 216

U.S.-source capital losses (even though the individual is not considered a resident of the U.S.), provided that the non-U.S. holder has timely filed U.S.
federal income tax returns with respect to such losses.

Determining whether we are a USRPHC depends on