HDAC9 and mature T-cell and NK-cell non-Hodgkin lymphoma: Several small-molecule HDAC inhibitors (HDACis) have been approved, such as vorinostat (SAHA) for CTCL, romidepsin for CTCL and PTCL, belinostat for PTCL, and chidamide for PTCL [3–7], making HDACs as the most promising and successful target in epigenetic regulators [8].