Histone deacetylase inhibitors (HDACIs) represent a new mechanistic class of anti-cancer therapeutics that target HDAC enzymes and have been shown to: arrest growth of cancer cells (including drug resistant subtypes), induce apoptosis, promote differentiation, inhibit angiogenesis, and sensitize cancer cells to overcome drug resistance when used in combination with other anti-cancer agents. This evidence concerns the gene HDAC9 and cancer.