HDAC9 and Thrombocytopenia: HDACi are hydroxamic acids, benzamides, cyclic peptides, ketones or aliphatic acids, and each HDACi targets different isoforms of HDAC.9 Recent studies have shown that HDACi are promising anti-tumor agents in various malignancies and other diseases.10 In MM, we have shown that non-selective HDACi induce anti-MM activities in preclinical settings;11, 12, 13 however, their clinical activities are limited due to unfavorable toxicities including fatigue, diarrhea and thrombocytopenia attendant to broad inhibition of HDAC isoforms.14