 Embryonic stem cells and imprinted genes are associated with both activating (H3K4me3) and repressive (H3K27me3) marks, as they allow a gene to be repressed until activation is needed.Although there is abundant evidence for co-localization of H3K4me3 and H3K27me3 at the same location in the genome, most evidence suggests that they do not occur on the same molecule but may occur on different copies of histone H3 within the same nucleosome.Bivalent chromatin domains are found in embryonic stem (ES) cells and play an important role in cell differentiation.