Allele-specific gene expression in mammals: the curious case of the imprinted RNAs.

Mammals exhibit two epigenetic phenomena whose consequence is the silencing of one of two wild-type copies of a gene X chromosome inactivation in females and parental imprinting of selected autosomal genes. As the mechanisms underlying these forms of gene dosage control are being elucidated, some striking similarities between them are being revealed. Of these similarities, none is more curious than the involvement of two noncoding RNAs in these processes, Xist, in the case of X chromosome inactivation, and H19, an imprinted RNA on chromosome 7. This article will explore the potential evolutionary relationship between X chromosome inactivation and imprinting by focusing on these unusual RNAs.