Genomic imprinting is the mechanism by which monoallelic expression is achieved in a parent-of-origin-specific fashion. At least 60 human genes are known to be imprinted (geneimprint.org) and can be further classified as “gametic” when the imprints are established in the germline or “somatic” when they arise during early embryonic development as a result of spreading of gametic imprints (reviewed in (John and Lefebvre, 2011)). Genomic imprints are particularly susceptible to environmental factors (Dolinoy et al., 2007; Odom and Segars, 2010) and imprinting defects are associated with developmental disorders, including Silver-Russell, Beckwith-Wiedemann, and Prader-Willi syndromes, as well as several human cancers (Bhusari et al., 2011; Uribe-Lewis et al., 2011). Variability in imprinting status has been reported for hPSCs (Adewumi et al., 2007; Frost et al., 2011; Kim et al., 2007; Rugg-Gunn et al., 2007), but the extent of this variation is unclear due to the limited number of imprinted genes, cell lines and cell types assayed in those studies.
