STAT5B and T-cell large granular lymphocyte leukemia: At variance STAT5b mutations have been reported to have a different clinical impact depending on which subset they were found, having been described to be associated to CD4+ T-LGL leukemia, characterized by indolent disease, [13] but having also been found in two cases of the rare aggressive form of LGL leukemia disorder (one with NK and the other with T type) [12].