Abstract
Genomic imprinting is an epigenetic phenomenon in which genes are expressed monoallelically in a parent-of-origin-specific manner. Each chromosome is imprinted with its parental identity. Here we will discuss the nature of this imprinting mark. DNA methylation has a well-established central role in imprinting, and the details of DNA methylation dynamics and the mechanisms that target it to imprinted loci are areas of active investigation. However, there is increasing evidence that DNA methylation is not solely responsible for imprinted expression. At the same time, there is growing appreciation for the contributions of post-translational histone modifications to the regulation of imprinting. The integration of our understanding of these two mechanisms is an important goal for the future of the imprinting field. This article is part of a Special Issue entitled: Chromatin and epigenetic regulation of animal development.