The chromosomal region, 15q11-q13, involved in Prader-Willi and Angelman syndromes (PWS and AS) represents a paradigm for understanding the relationships between genome structure, epigenetics, evolution, and function. The PWS/AS region is conserved in organization and function with the homologous mouse chromosome 7C region. However, the primate 4 Mb PWS/AS region is bounded by duplicons derived from an ancestral HERC2 gene and other sequences that may predispose to chromosome rearrangements. Within a 2 Mb imprinted domain, gene function depends on parental origin. Genetic evidence suggests that PWS arises from functional loss of several paternally expressed genes, including those that function as RNAs, and that AS results from loss of maternal UBE3A brain-specific expression. Imprinted expression is coordinately controlled in cis by an imprinting center (IC), a genetic element functional in germline and/or early postzygotic development that regulates the establishment of parental specific allelic differences in replication timing, DNA methylation, and chromatin structure.