HDAC9 and mature T-cell and NK-cell non-Hodgkin lymphoma: Although HDAC inhibitors have been approved for relapsed/refractory T cell lymphomas, including PTCL and CTCL [33], this is the first successful application of the HDAC inhibitor chidamide in a focused study of relapsed/refractory NKTL in which the underlying drug resistance mechanisms in this disease have been investigated and reported.