Normal human development involves a delicate interplay of gene expression in specific tissues at narrow windows of time. Temporally and spatially regulated gene expression is controlled both by gene-specific factors and chromatin-specific factors. Genomic imprinting is the expression of specific genes primarily from only one allele at particular times during development, and is one mechanism implicated in the intricate control of gene expression. Two human genetic disorders, Prader–Willi syndrome (PWS, MIM 176270) and Angelman syndrome (AS, MIM 105830), result from rearrangements of chromosome 15q11-q13, an imprinted region of the human genome. Despite their rarity, disorders such as PWS and AS can give focused insight into the role of genomic imprinting and imprinted genes in human development.