Imprinting provides a fascinating mechanism of control of gene expression so that the maternal and paternal alleles of some genes are unequally expressed. Imprinting is most likely established during gametogenesis by a mechanism not completely clear, though DNA methylation probably plays a certain role. Expression of imprinted gene significantly affects mammalian development so that only the maternal or only the paternal diploid genomes cannot support normal development. Since imprinting results in functional hemizygocity, mutation of the expressed allele can have the drastic consequences of a null mutation. For this reason identification of imprinted genes and further understanding of the imprinting mechanism represent an important change for human medical genetics.