NDN and Prader-Willi syndrome: This syndrome is related to deletion of parental copies within the chromosome region 15q11-q13; more in detail, Prader–Willi syndrome (PWS) results from the deletion of the paternal copies of the imprinted SNRPN gene, the NDN gene, and possibly other genes, while Angelman syndrome (AS) is caused by de novo maternal deletions involving chromosome 15q11.2-q13 (70%), by paternal uniparental disomy of 15q11.2-q13 (2%), by imprinting defects (2–3%), and by mutations in the gene encoding the ubiquitin–protein ligase UBE3A gene (25%) [49].