Capsida Biotherapeutics' clinical trial testing a gene therapy for STXBP1 encephalopathy, a rare pediatric neurological disorder, suffered a significant setback after a patient died shortly following dosing. The company and investigators are investigating the event's cause. STXBP1 encephalopathy is characterized by seizures and developmental delays and remains difficult to treat. This incident underscores the challenges in developing gene therapies targeting complex brain disorders.