Approximately 5-10% of patients with Angelman syndrome have mutations in the UBE3A gene. This mechanism should be considered for patients that fit the classic AS phenotype yet have normal methylation of chromosome 15. Up to 50% of all patients without molecular confirmation of the other mechanisms have a mutation in UBE3A, including 75-80% of all familial cases in this category. The phenotype of these patients has been described as intermediate between those of the deletion group and the UPD/IC defect group; seizure frequency, speech impairment, and severity of microcephaly is similar to what is noted in the deletion group, while ability to develop of motor skills and obesity is similar to that in the UPD/IC group.
Deletions and/or duplications of the UBE3A gene as causative of Angelman syndrome have been reported.