MKI67 and mature T-cell and NK-cell non-Hodgkin lymphoma: Peripheral T cell lymphoma, not otherwise specified (PTCL-NOS) is a heterogeneous group of mature T cell lymphomas which do not correspond to any specific subtype of mature T-cell lymphoma in current classifications.[1] Some researchers, especially those in Japan, have suggested that PTCL with low Ki-67 labeling index (Ki-67 LI) should be classified as indolent PTCL.[2] These indolent PTCLs histologically show diffuse infiltrated small lymphoid cells with low KI-67 LI and have a relatively good prognosis, although the epidemiology and pathogenesis are not fully elucidated.