This is a Phase 1/2, multinational, open-label, study to evaluate the safety and efficacy of an intravenous infusion of GNT0003 in patients with Crigler-Najjar aged ≥10 years and requiring phototherapy. Patients will received a single administration of GNT0003 and will be followed for safety and efficacy of approximately 60 months (5 years):

• a follow-up of approximately 12 months (48 weeks)
• a long term follow-up of approximately 48 months (4 years), in order to be in line with the latest EMEA Guideline on follow-up of patients administered with gene therapy medicinal products, released on 22 Oct.2009 by the Committee for medicinal products for human use.

Inclusion Criteria:

• Patients with severe Crigler-Najjar syndrome resulting from a molecular confirmation of mutations in the UGT1A1 gene and requiring phototherapy
• Male or female at least 9 years at the date of signature of informed consent
• Patient able to give informed assent and/or consent in writing

Exclusion Criteria:

• Patients who underwent liver transplantation
• Patients with chronic hepatitis B or C
• Patients infected with Human immunodeficiency virus (HIV)
• Patients with significant underlying liver disease
• Patients with significant encephalopathy
• Participation in any other investigational trial during this trial
• Patients unable or unwilling to comply with the protocol requirements