Genomic imprinting is defined as the differential expression of a gene or chromosomal region according to the parental origin of inheritance. Both the maternal and paternal alleles are present, but while one is functionally active, the other is silenced (inactive) in somatic cells (Figure 1a). Genomic imprinting is reversible through successive generations. Inherited maternal and paternal “imprints” are erased during gametogenesis (gamete production) and new imprints established according to the sex of the parent. Genomic imprinting is an epigenetic phenomenon (not dependent on the DNA sequence itself, but rather on factors that regulate DNA activity). These factors include attachments of methyl groups to the DNA, which are set in the gamete and serve to distinguish the parental alleles in somatic cells post-fertilization.