Prader-Willi syndrome (PWS) is a disease caused by mutations on human chromosome 15 leading to "floppy" infants initially, and obesity and sleep disorders later. Although genetic defects underlying PWS have been documented, it is still not well understood how the loss-of-function of genes results in various symptoms in PWS. It has been shown that the dysfunction of the hypothalamus, a brain structure, contributes to symptoms seen in PWS patients.