HDAC9 and mature T-cell and NK-cell non-Hodgkin lymphoma: Chidamide is benzamide class novel HDAC inhibitor and is approved by China food and drug administration (CFDA) for treating those suffering from refractory/relapsed PTCL.20 Our previous results showed that PD‐1 expression on peripheral blood lymphocytes was reduced in patients who responded to treatment of chidamide combined with chemotherapy,16 indicating that chidamide may exert its anti‐tumor potential by regulating PD‐1 (+) cell function.