An autosomal dominant disorder caused by deletion of the proximal long arm of the paternal chromosome 15 (15q11-q13) or by inheritance of both of the pair of chromosomes 15 from the mother (uniparental disomy) which are imprinted (GENETIC IMPRINTING) and hence silenced. Clinical manifestations include MENTAL RETARDATION; MUSCULAR HYPOTONIA; hyperphagia; obesity; short stature; hypogonadism; strabismus; and HYPERSOMNOLENCE. (Menkes, Textbook of child neurology, 5th ed, p229)