Despite the capability of STAT5AS710F but also STAT5AS710A to rescue STAT5A−/− T cell proliferation, only STAT5AS710F was shown to result in the induction of leukemia upon expression in the bone marrow of wild-type and STAT5−/− mice, demonstrating that the hydrophobic properties of phenylalanine are required for the oncogenic potential of STAT5AS710F. Here, STAT5B is linked to leukemia.