Our observation of activating mutations in JAK3 and STAT5B in T-PLL and HSTL, along with these prior studies, show that mutations leading to activation of γc associated JAK-STAT cytokine signaling pathways are present in at least 5 subtypes of PTCL and may represent a unifying trait in a disease marked by heterogeneity between subtypes. The gene discussed is STAT5B; the disease is mature T-cell and NK-cell non-Hodgkin lymphoma.