Passage: PWS is caused by an epigenetic phenomenon known as imprinting, caused by the deletion of the paternal copies of the SNRPN and NDN necdin genes along with clusters of snoRNAs: SNORD64, SNORD107, SNORD108 and two copies of SNORD109, 29 copies of SNORD116 (HBII-85) and 48 copies of SNORD115 (HBII-52). These are on chromosome 15 located in the region 15q11-13. This so-called PWS/AS region may be lost by one of several genetic mechanisms, which in the majority of instances occurs through chance mutation. Other, less common mechanisms include uniparental disomy, sporadic mutations, chromosome translocations, and gene deletions. Due to imprinting, the maternally inherited copies of these genes are virtually silent, and only the paternal copies of the genes are expressed. PWS results from the loss of paternal copies of this region. Deletion of the same region on the maternal chromosome causes Angelman syndrome (AS). PWS and AS represent the first reported instances of imprinting disorders in humans.

Question: What is the cause of PWS?
phenomenon known as imprinting