Genomic imprinting: a mammalian epigenetic discovery model.

Genomic imprinting is an epigenetic process leading to parental-specific expression of one to two percent of mammalian genes that offers one of the best model systems for a molecular analysis of epigenetic regulation in development and disease. In the twenty years since the first imprinted gene was identified, this model has had a significant impact on decoding epigenetic information in mammals. So far it has led to the discovery of long-range cis-acting control elements whose epigenetic state regulates small clusters of genes and of unusual macro noncoding RNAs (ncRNAs) that directly repress genes in cis, and critically, it has demonstrated that one biological role of DNA methylation is to allow expression of genes normally repressed by default. This review describes the progress in understanding how imprinted protein-coding genes are silenced; in particular, it focuses on the role of macro ncRNAs that have broad relevance as a potential new layer of regulatory information in the mammalian genome.