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Upfront payment.

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The cash inflow associated with the amount received from entity's raising of capital via private rather than public placement.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Astellas Agreements - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 21, 2022	Oct. 31, 2022	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, shares issued			287,276,885		285,051,648
Probability of option exercise		50.00%
Revenues			$ 0	$ 2,302
Rett Option
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues					$ 5,500
Rett research and development activities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues				$ 2,300
Astellas
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront Payment	$ 20,000
Transaction price		$ 36,100	36,059
Deferred revenue		$ 36,100
Astellas \| Securities Purchase Agreement \| Private Placement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, shares issued	7,266,342	7,266,342
Aggregate gross proceeds	$ 30,000
Fair value of shares transferred, Value		$ 13,950
Share price		$ 1.92
Transaction price		$ 16,100
Difference of market value paid		$ 30,000
Astellas \| TSHA-120
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront Payment			20,000
Transaction price			36,100
Astellas \| TSHA-120 \| Private Placement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Transaction price			16,100
Astellas \| Rett research and development activities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Transaction price			$ 28,257