Praer-Willi syndrome (abbreviated PWS) is a very rare genetic disorder, in which seven genes (or some subset thereof) on chromosome 15 are missing or unexpressed (chromosome 15q partial deletion) on the paternal chromosome. It was first described in 1956 by Andrea Prader, Heinrich Willi, Alexis Labhart, Andrew Ziegler, and Guido Fanconi of Switzerland. The incidence of PWS is between 1 in 12,000 and 1 in 15,000 live births. The distinction of chromosome by parental origin is due to imprinting and PWS has the sister syndrome Angelman syndrome that affects maternally imprinted genes in the region.