Angelman syndrome (AS) and Prader–Willi syndrome (PWS) are distinct clinical phenotypes resulting from maternal and paternal deficiencies, respectively, in human chromosome 15q11–q13. Although several imprinted, paternally expressed transcripts have been identified within the PWS candidate region, no maternally expressed gene has yet been identified within the AS candidate region. We have developed an integrated physical map spanning the PWS and AS candidate regions and localized two breakpoints, including a cryptic t(14;15) translocation associated with AS and a non-AS 15q deletion, which substantially narrow the AS candidate region to ∼250 kb. Mapping data indicate that the entire transcriptional unit of the E6–AP ubiquitin–protein ligase (UBE3A) gene lies within the AS region. TheUBE3Alocus expresses a transcript of ∼5 kb at low to moderate levels in all tissues tested. The mouse homolog ofUBE3Awas cloned and sequenced revealing a high degree of conservation at nucleotide and protein levels. Northern and RT–PCR analysis ofUbe3aexpression in mouse tissues from animals with segmental, paternal uniparental disomy failed to detect substantially reduced or absent expression compared to control animals, failing to provide any evidence for maternal-specific expression from this locus. Recent identification of de novo truncating mutations inUBE3Ataken with these observations indicates that mutations inUBE3Acan lead to AS and suggests that this locus may encode both imprinted and biallelically expressed products.[The sequence data described in this paper have been submitted to the GenBank data library under accession no.U82122.]