The invention provides a polynucleotide sequence (e.g., a gene, e.g., DNA or RNA) encoding UGT1A1 (e.g., expressing human UGT1A1). The invention further provides a vector, such as an adeno-associated virus (AAV) vector (e.g., AAV8) having a vector genome including inverted terminal repeat sequences and a UGT1A1 coding sequence operably linked to one or more expression control sequences (e.g., expression control sequences including a liver-specific promoter). Also provided are compositions containing these AAV vectors and methods of treating Crigler-Nijjar syndrome type I, Crigler-Nijjar syndrome type II, and Gilbert syndrome.