By contrast to AS, which is caused by a paternal duplication of the chromosome 15q11-q13 and/or a UBE3A mutation [24,26,27], PWS, another neurodevelopmental genomic imprinting disorder, is caused by its maternal duplication, resulting in overexpression of UBE3A. The gene discussed is UBE3A; the disease is Prader-Willi syndrome.