Angelman syndrome is associated with ataxia, seizures, absence of speech, hyperactivity, hypopigmentation, and severe mental retardation, and is caused by absence of the maternally-inherited criticial region of chromosome 15.

For diagnosis, DNA methylation study of the PWS/AS Critical Region is the recommended method. If the methylation pattern is characteristic of paternal inheritance, this is diagnostic for Angelman syndrome. Paternal inheritance of the PWS/AS critical region can be caused by deletions of the maternal allele, receiving two copies of chromosome 15 from the father (uniparental disomy) or defects in the imprinting process.

DNA methylation studies will detect ~78% of cases of Angelman syndrome. If the methylation test is positive, a FISH (fluorescence in situ hybridization) test can be used to distinguish between uniparental disomy and a deletion.

11% of individuals with a clinical diagnosis of Angelman syndrome will have an imprinting defect caused by mutations in the UBE3A gene. Individuals with a clinical diagnosis of Angelman but normal methylation studies should have DNA sequencing of the UBE3A gene.