NDN and Angelman syndrome: At the molecular level, patUPD15 and imprinting defects caused by maternal PWS-IC loss of methylation differ from other Angelman syndrome cases due to the abnormal biallelic expression of several paternally expressed imprinted genes (NDN, MAGEL2, MKRN3, SNRPN/SNURF, SNORD115, SNORD116) alongside UBE3A loss.