Despite the STAT5B hyperactivation and the presence of multiple STAT5B mutations, the clinical course of CD4+ T-LGLL is indolent, and patients rarely have symptoms, differentiating it from CD8+ T-LGLL and other mature and immature CD4+ T-cell leukemias and lymphomas. The gene discussed is STAT5B; the disease is T-cell large granular lymphocyte leukemia.