Restricted repetitive behavior (RRB) often causes significant disruption to the daily lives of the affected individuals and their families. Despite its significant functional impact, little is known about underlying genetic mechanisms of RRB in general. Prader-Willi syndrome (PWS) is a rare genetic disorder caused by absence of paternally inherited genes in the 15q11-q13 chromosomal region. Interestingly, the 15q11-q13 region has been also implicated in subset of the autism spectrum disorder (ASD) population. Furthermore, children with ASD and children with PWS show similar levels of RRB, suggesting at least a part of RRB phenotypes of both PWS and ASD may be associated with a common neuropsychological, neurotransmitter, or genetic origin. This study will use Prader-Willi syndrome and autism spectrum disorder as models to study genetic mechanisms of restricted repetitive behavior (RRB), focusing on 15q11-q13 region. Identification of common genetic elements in these clinical groups will provide insights into disease pathogenesis and help identify novel targets for treatment of RRB.