After analysis of its selectivity profile, the most potent inhibitor was then developed to Stafia-1, the first small molecule shown to preferentially inhibit the STAT family member STAT5aover the close homologue STAT5b. A phosphonate prodrug based on Stafia-1 inhibited STAT5a with selectivityoverSTAT5b in human leukemia cells, providing the first demonstration of selective in vitro and intracellular