NDN and Prader-Willi syndrome: The genetic causes of PWS include paternal 15q11.2-q13 deletions, maternal uniparental disomy (UPD) 15, and imprinting defects (3), all leading to loss of expression of the paternal-only expressed genes MKRN3, MAGEL2, NECDIN, SNURF, SNRPN and seven non-coding RNAs (1).