HDAC inhibitors (HDACi) are the most developed anti-cancer drugs targeting epigenetic modulation and are being applied for the treatment of various cancers, particularly in solid tumors, such as breast, colon, lung, and ovarian cancers, as well as in haematological tumors, such as lymphoma, leukemia, and myeloma [6–9]. This evidence concerns the gene HDAC9 and leukemia.