STAT5B and prolymphocytic leukemia: Accordingly, activating somatic mutations in the SH2 domain and the following C-terminal tail segment (CTS) of STAT5 (Fig. 1a) have been discovered in distinct types of T- and NK-cell lymphomas such as large granular lymphocytic leukemia (LGLL), acute T-cell leukemia (T-ALL) and T-cell prolymphocytic leukemia (T-PLL)14, 15, 16, 17, 18, 19.