Another member of STAT protein family has been reported to carry gain-of-function mutations, namely STAT5b. Initially discovered in only 2% of CD8+ T-LGLL, specifically found in the aggressive form of LGLL (28), STAT5b mutations were subsequently identified in 15–55% of CD4+ T-LGLL (14, 15, 29), and in 19% of TCRγδ LGLL (15). Here, STAT5B is linked to T-cell large granular lymphocyte leukemia.