STAT5B mutation is over-represented in the rare CD4+ T-LGLLs (present in up to 30% of cases); it is associated with a poor prognosis in CD8+ T-LGLL, but has no prognostic impact in CD4+ T-LGLL and gamma/delta T-LGLL [214]. The gene discussed is STAT5B; the disease is T-cell large granular lymphocyte leukemia.