Segment Information - Information On The Segment And Reconciliation To Net Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Segment Reporting, Asset Reconciling Item [Line Items]
Revenue under licensing agreements	$ 0	$ 925
Total research and development expenses	75,011	60,917
Total general and administrative expenses	28,317	22,613
Other income	12,977	7,461
Net loss	(90,351)	(75,144)
Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total research and development expenses	75,011	60,917
Total general and administrative expenses	28,317	22,613
Net loss	(90,351)	(75,144)
Rett syndrome | Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total research and development expenses	25,841	12,104
Batten disease | Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total research and development expenses	1,513	5,869
Early Discovery | Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total research and development expenses	3,223	5,401
Personnel-related | Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total research and development expenses	21,493	18,476
Total general and administrative expenses	9,257	8,098
Stock-based compensation | Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total research and development expenses	6,472	4,506
Total general and administrative expenses	7,711	3,813
Manufacturing | Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total research and development expenses	13,018	12,098
Other | Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total research and development expenses	3,451	2,463
Total general and administrative expenses	4,298	3,610
Professional and consultant fees | Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total general and administrative expenses	4,586	4,558
Office-related | Reportable Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total general and administrative expenses	$ 2,465	$ 2,534