Key Inclusion Criteria: * Subject has a diagnosis of Crigler-Najjar syndrome resulting from a confirmed mutation in the UGT1A1 gene as assessed by a Sponsor-approved testing facility. * Subject is aged ≥1 year. * Subject is prescribed daily phototherapy for a minimum of 6 hours within a 24-hour period (daily illumination time). Key Exclusion Criteria: * Subject is currently participating in an interventional study or has received gene or cell therapy. * Subject has received a whole liver, partial liver, or hepatocyte transplant; or subject has a liver transplant scheduled within the treatment period of this study. * Subject has significant cholestatic disease at screening. * Subject is receiving phenobarbital or other known inducer of UGT1A1 within 30 days of screening. * Subject tests positive for AAV8 neutralizing antibodies with titers above protocol specified threshold. * Other than as required per protocol, subject has received immune-modulating agents within 3 months before dosing (use of inhaled corticosteroids to manage chronic respiratory conditions is allowed); use of other concomitant medications to manage chronic conditions must have been stable for at least 4 weeks before dosing. * Subject has any clinically significant laboratory values, in the opinion of the investigator. * Subject has clinically significant underlying liver disease (other than CN) at screening. * Subject has a history of, or currently has, a clinically important condition other than CN, in the opinion of the investigator.