Conversely, STAT5B mutations, initially discovered in CD8 + T-LGLL with dismal outcome [10], are found at high frequency in CD4 + T-LGLL patients, which mostly present with indolent disease [6, 11, 12], similarly to STATs-mutation negative T-LGLL cases [6]. Here, STAT5B is linked to T-cell large granular lymphocyte leukemia.