Genomic imprinting refers to a phenomenon whereby one of the two alleles at a gene locus is preferentially expressed depending upon the parent of origin. Imprinting manifests at the level of the genome, the individual chromosome/chromosomal region and the gene. For many years it has been recognized in the mouse, and more recently has been identified in humans. The mechanisms of imprinted gene expression are not fully understood, but deoxyribonucleic acid (DNA) methylation and asynchronous replication of the parental alleles (allele-specific replication) are common characteristics of imprinted genes. DNA methylation and allele-specific DNA replication are involved in both the somatic cell expression and inheritance of the imprint. Abnormalities related to genomic imprinting have been observed in several human disorders, marked by alterations in prenatal and/or postnatal growth. The best-characterized disorders that result from abnormal genomic imprinting include triploidy, complete hydatidiform moles (a condition in which the placenta degenerates and the fetus is reabsorbed), ovarian teratomas, and the Beckwith-Wiedemann, Angelman, and Prader-Willi syndromes.