The
epigenetic events that occur during the development of the mammalian embryo are essential for correct gene expression
and cell-lineage determination. Imprinted genes are expressed from only one parental allele due to differential
epigenetic marks that are established during gametogenesis. Several theories have been proposed to explain the role that genomic
imprinting has played over the course of mammalian evolution, but at present it is not clear if a single hypothesis can fully account for the diversity of roles that imprinted genes play. In this review, we discuss efforts to define the extent of imprinting in the mouse genome, and suggest that different imprinted loci may have been wrought by distinct evolutionary forces. We focus on a group of small imprinted domains, which consist of paternally expressed genes embedded within introns of multiexonic transcripts, to discuss the evolution of imprinting at these loci.