Capsida Biotherapeutics has paused enrollment in its Phase I/II Synrgy clinical trial for STXBP1 encephalopathy, a rare pediatric neurodevelopmental disorder, following the death of the first patient treated with its gene therapy CAP-002. The company and clinical investigators are urgently investigating the cause. This setback raises critical questions about safety in emerging gene therapy approaches for neurological disorders and the mechanisms needed to mitigate risk in pediatric trials.