The activating STAT5B mutation, N642H, which is observed repeatedly in PTCL, leads to a more aggressive clinical course in T-LGL and a significantly reduced probability of event free survival and an increase in cumulative incidence of relapse in T-cell acute lymphoblastic leukemia (T-ALL) [11, 35]. The gene discussed is STAT5B; the disease is T-cell large granular lymphocyte leukemia.