Of interest to our research, activating STAT5 mutations have been observed in multiple PTCL subtypes at frequencies of 21-36% in T-cell prolymphocytic leukemia (T-PLL), 33% in hepatosplenic T-cell lymphoma (HSTL), 63% in epitheliotropic intestinal T-cell lymphoma (EITL), 3% in Sézary syndrome (SS), and 2% in T-cell large granular lymphocytic leukemia (T-LGL) [11, 15, 20, 22–24]. Here, STAT5B is linked to T-cell prolymphocytic leukemia.