Abstract
Studies of large imprinted clusters, such as the Gnas locus, have revealed much about the significance of DNA methylation, transcription and other factors in the establishment and maintenance of imprinted gene expression. However, the complexity of such loci can make manipulating them and interpreting the results challenging. We review here a distinct class of imprinted genes, which have arisen by retrotransposition, and which have the potential to be used as models for the dissection of the fundamental features and mechanisms required for imprinting. They are also of interest in their own right, generating diversity in the transcriptome and providing raw material upon which selection can act.