HDAC9 and mature T-cell and NK-cell non-Hodgkin lymphoma: To date, four HDAC inhibitors have been approved for the treatment of relapsed PTCL—class I, II, and IV inhibitors (Vorinostat (SAHA), Belinostat), the selective HDAC1, 2, 3, and 10 inhibitor chidamide, and the selective class I inhibitor Romidepsin.