A covalent JAK3 inhibitor (IC50 = 33.1 nM); selective for JAK3 over JAK1, JAK2, and TYK2 (IC50s = >10,000 for all) and a panel of 304 other kinases at 1 µM; inhibits STAT5, STAT6, and STAT3 phosphorylation induced by IL-2, IL-4, or IL-21, respectively, in isolated human lymphocytes (IC50s = 244, 340, and 350 nM, respectively); inhibits T cell differentiation into Th1 and Th17 in a T cell differentiation assay (IC50s = 30 and 167 nM, respectively); reduces paw swelling in a rat model of adjuvant-induced arthritis at 3, 10, or 30 mg/kg; decreases disease severity in a mouse model of EAE when at 30 or 100 mg/kg