gene in the gamete inherited from either the mother or the father, but not both (i.e., allele-specific silencing). The best studied silencing mechanism is methylation, and differential methylation between alleles is considered the hallmark feature of an imprinted locus127. The cellular mechanisms for sex-specific gene silencing and the impact of such parent-of-origin effects on human disease and gene evolution have been previously reviewed127-130.In roughly half of imprinted genes the maternally-inherited allele is silenced (i.e., imprinted) and in the other half the paternally-inherited allele is silenced. In a few interesting cases, the imprinting itself is polymorphic so that both bi-allelic and monoallelic expression is observed between individuals131,132. Mutations in or deletions of the expressed allele at imprinted loci in humans or mice have a wide range of phenotypic consequences, including effects on growth and development, behavior and learning, and carcinogenesis127,128.A census of imprinted genes in 2005 suggested that approximately 41 genes in 16 chromosomal regions are imprinted in humans, compared to 71 genes in 22 chromosomal regions in mice (29 of the same genes are imprinted in both humans and mice)130. The authors speculated that the total numbers of imprinted genes are probably not much greater than these estimates, although they
