Although recent studies have revealed activating STAT3 mutations in some malignancies (hepatocellular adenoma, 40% of large granular lymphocytic leukemia, and 30% of chronic lymphoproliferative disease of NK cells), these mutations are too rare to account for the high prevalence of STAT3 activation in solid tumors. The gene discussed is STAT3; the disease is T-cell large granular lymphocyte leukemia.