Figure 4. Both H3K4me3 and H3K27me3 marks are critical for the maintenance and differentiation of stem cells. H3K4me3 is the epigenetic mark for transcriptionally active chromatin, and H3K27me3 is the epigenetic mark for transcriptionally inactive chromatin. In stem cells and progenitor cells, many genes important for developmental control are marked by both H3K4me3 and H3K27me3 marks and therefore are poised for transcription activation or repression. Thus, the enzymes that regulate the levels of H3K4me3 and H3K27me3 are important for the homeostasis of the stem cells.