Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 324

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 324
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 considered unobservable:

Level1—Quoted market prices in active markets for identical assets or
liabilities.

Level2—Inputs other than Level 1 inputs that are either directly or
indirectly observable, such as quoted market prices, interest rates and yield curves.

Level3—Unobservable inputs for the asset or liability (i.e., supported by little or no market
activity). Level 3 inputs include management’s own assumptions about the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk).

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of the fair
value requires more judgement. 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 input that is significant to the fair value measurement.

F-9

Property and Equipment, net Property and equipment is stated at cost net of accumulated depreciation. Costs of major additions and betterments are capitalized. Maintenance and repairs to an asset that do not improve or extend its life are expensed in the period incurred. Depreciation expense is recognized using the straight-line method over the estimated useful life of each asset as follows:

|                        |     |               Estimated Useful Life (in years) |
| Lab equipment          |     |                                              5 |
| Furniture & fixtures   |     |                                              5 |
| Hardware & software    |     |                                              3 |
| Leasehold improvements |     | Shorter of useful life or remaining lease term |

Construction-in-progressis stated at cost, which includes direct costs attributable to the construction of the related asset. Depreciation expense is not recorded on construction-in-progressuntil the relevant assets are completed and placed into service. When an item is sold or retired, the costs and related accumulated depreciation are eliminated, and the resulting gain or loss, if any, is credited or charged to the consolidated statement of operations and comprehensive loss. Property and equipment to be disposed of are carried at fair value less costs to sell. Impairment of Long-Lived Assets Long-lived assets consist primarily of property and equipment and right-of-useassets. The Company continually evaluates whether events or circumstances have occurred that indicate that the estimated remaining useful life of its long-lived assets may warrant revision or that the carrying value of these assets may be impaired. An impairment loss would be recognized when estimated undiscounted future cash flows expected