Supportive evidence has included intervention application of epigenetic modulators such as DNA methyltranferase (DNMT) inhibitor, 5-aza-2’-deoxycytidine (5-aza), and histone deacetylase (HDAC) inhibitor, trichostatin A (TSA), which successfully induced ER expression and sensitized hormone-resistant ERα-negative breast cancer cells to chemotherapy [13-16]. Here, HDAC9 is linked to breast cancer.