Prader–Willi syndrome is a rare genetic disorder in which seven genes (or some subset thereof) on chromosome 15 (q 11–13) are deleted or unexpressed (chromosome 15q partial deletion) on the paternal chromosome. Characteristic of PWS is low muscle tone, short stature, incomplete sexual development, cognitive disabilities, problem behaviours, and a chronic feeling of hunger that can lead to excessive eating and obesity. The incidence of PWS is between 1 in 25000-10000 live births.
Sequencing studies help pinpoint gene in Prader-Willi Syndrome (via Baylor college of Medicine). Also here (subscription required): http://www.nature.com/ng/journal/vaop/ncurrent/ng.2776/metrics and http://www.eurekalert.org/pub_releases/2013-09/bcom-ssh092413.php