Studies concluded that treatment of cancer cells with histone deacetylation (HDAC) inhibitor and PARP inhibitor had a synergistic effect because of the induction of HDAC inhibitor on homologous recombination deficiencies, which further sensitized the cancer cells to PARP inhibitor (Jasek et al., 2014; Baldan et al., 2015; Ha et al., 2014; Min et al., 2015). Here, HDAC9 is linked to cancer.