A potent JAK3 inhibitor (IC50 = 127 pM); 400-, 2,700-, and 3,600-fold selective for JAK3 over JAK1, JAK2, and TYK3 and only inhibits 11 kinases in a panel of 410 kinases at 500 nM; blocks JAK3-dependent STAT5 phosphorylation but not JAK3-independent STAT3 signaling in IL-2- and IL-6-stimulated T cells, respectively, at 1 μM,