Histone modification patterns at imprinted genes across cell types. Histone modification profiles for H3K4me3 and H3K27me3 were analysed at transcription start sites of imprinted genes (n = 54) in mouse (A) embryonic stem cells (ESCs), (B) neural progenitor cells (NPCs) and (C) mouse embryonic fibroblasts (MEFs) using Mikkelsen et al.  whole-genome source data. The patterns of enrichment shown differ significantly across the three cell types (chi-square contingency test, P < 0.0005). Coenrichment of H3K4me3 and H3K27me3 is significantly higher at ESCs compared to NPCs (Yates' chi-square test, P < 0.01). A reduction from 41% in ESCs to 22% in MEFs is observed for this specific profile (Yates' chi-square test, P = 0.123).