Crigler-Najjar Syndrome (CN) is a rare genetic disorder caused by a defect of the UGT1A1 gene, which encodes the protein responsible for metabolizing bilirubin. The resulting toxic accumulation of bilirubin can cause severe neurological impairment and death. Phototherapy can help manage bilirubin levels, but requires 10-12 hour treatments each day and becomes less effective with age, at which point a liver transplant is required. GTP and Audentes Therapeutics are collaborating to advance AAV8-UGT1A1 (AT342) for the treatment of CN. A phase 1/2, multinational, open-label, ascending-dose, delayed-treatment concurrent control clinical study is planned (VALENS; NCT03223194).