Prader-Willi Syndrome (PWS), a complex genetic disorder that results from a failure to inherit a normal copy of the father’s chromosome 15. Key genes on the paternal chromosome are deleted, but how the deletion of this region leads to PWS is a complete mystery. In PWS, a normal copy of the mother’s chromosome 15 is inherited, but the genes are turned off due to what’s called genomic imprinting. This genomic imprinting is a normal process in development that turns off the maternal genes. Not much is known about the mechanisms by which the maternal chromosome is turned off in humans. The studies proposed here will provide important new information about how the region is regulated, which is a necessary step to set the stage for a better understanding of the PWS disease and, eventually, for the development of new therapeutic strategies. Our preliminary experiments in induced pluripotent stem cells derived from the skin of PWS patients suggest that there are novel mechanisms that regulate the expression of the maternal chromosome.