HDAC9 and neoplasm: Mechanistically, PARP/HDACi increased the acetylation of H3K9 and H2A.X phosphorylation and reduced the expression of RAD51 and BRAC1 on tumor cells in MDA-MB-436 and 4T1 mice (Fig. 4G, H and Fig. S1G), suggesting that the antitumor effect in vivo coincides with disruption of PARP and HDAC enzymatic activity.