TP53BP1 and breast cancer: Dimethylation of both H4K20 and H3K79 has an important and direct role in the DDR because it has been demonstrated to modulate the recruitment of 53BP1 and its focus formation.4, 5 Indeed, the HMT-induced decreases in H4K20 and H3K79 dimethylation were accompanied by a significant decrease in the IR-damage-induced focus formation of 53BP1, but not of γH2AX, in BC cells (Figure 2c).