HDAC inhibitors can reactivate the expression of tumor suppressors and have emerged as one type of well-characterized epigenome-targeting agents that are capable of suppressing both histone acetylation and non-histone acetylation to resolve tumors [110], halt cancer metastasis [111], reprogram cancer cell metabolism [112,113], modulate the immune [114], chemo [115] and radio [116] sensitivity of cancer cells. This evidence concerns the gene HDAC9 and neoplasm.