Most NK-LGLL patients harbor no STAT5B mutation.[15–17] Only few report showed that Y655F or N642H mutation in STAT5B have been identified in 2 patient with NK-LGLL respectively.[18,19] Interestingly, a study found that STAT5B mutation impaired human NK cell maturation which leads to autoimmunity, recurrent infections, and combined immune deficiency.[20] Pathogenic N642H mutation of STAT5B was defined in our patient, which serve as a proof to differentiate from reactive NK cell proliferation. Here, STAT5B is linked to T-cell large granular lymphocyte leukemia.