HDAC9 and diffuse large B-cell lymphoma: Of note, epigenetic alterations can be reversed by pharmacological drugs, such as histone deacetylase (HDAC), DNA methyltransferase (DNMT), or histone methyltransferase (HMT) inhibitors, and they are currently used in the clinic or tested in clinical trials in patients with relapsed or refractory DLBCL [16–19].