XRCC6 and esophageal squamous cell carcinoma: Valproic acid (VPA) induces high acetylation of histones H3, H4 and Ku70 and apoptosis and prolongs IR-induced DSBs by inducing downregulation of the DNA repair protein Rad51 to inhibit HR in the DSB repair pathway in ESCC, thereby enhancing radiation-induced cytotoxicity in human ESCC cells [161].