Histone modification patterns in mouse embryonic stem cells (ESCs). Profiles for H3K4me3, H3K27me3 and H3K9me3 were analysed in mouse ESCs at transcription start sites of (A) imprinted genes (n = 54) and (B) all genes (n = 17,761) using source enrichment data of Mikkelsen et al. . 'Other' represents alternative combinations of these three histone modifications and 'None' represents genes without any of these three modifications. The data does not meet the conditions of the chi-square test when comparing all profiles and also when specifically comparing H3K4me3 and H3K9me3 between imprinted and all genes. H3K4me3 and H3K27me3 are together enriched more often at imprinted genes than all genes (Yates' chi-square test, P < 0.005).