Epigenetic mechanisms, such as histone modifications, control eukaryotic development beyond DNA-stored information. Intrigu-ingly, there is an under-representation of repressive marks in quies-cent (resting) cells, stem cells and regenerating cells, but a selective accumulation of aberrant histone lysine methylation profiles in aging, ‘‘stressed’’ and tumor cells, particularly for the H3K9, H3K27 and H4K20 methyl marks.