HDAC9 and neoplasm: Cell signaling targeted therapies (HDAC, PI3K/AKT/mTOR, p38 MAPK, Hsp90, Wnt, Notch, Hedgehog, and cell cycle) and strategies targeting the tumor microenvironment (hypoxia, angiogenesis, integrins, CD44, CXCR4, and selectins) have yielded promising results alone or in combinations in preclinical or clinical studies involving patients with relapsed/refractory MM.