UBE3A and Angelman syndrome: Angelman syndrome (AS) is a severe genetic neurodevelopmental disease secondary to the loss of function of E6-AP ubiquitin ligase (UBE3A) gene caused by one of these four mechanisms: deletion of chromosome 15q11–q13 region of maternal origin (75%), paternal uniparental disomy (5-10%), an imprinting defect (5-10%) or point mutation in the maternal origin allele of UBE3A [27].