CP-690,550 is a potent and selective JAK inhibitor currently in clinical trials for rheumatoid arthritis (RA) and other autoimmune disease indications. In enzyme assays it potently inhibited recombinant human kinase protein JAK1 (IC50=3.2nM), JAK2 (IC50=4.1nM) and JAK3 (IC50=1.6nM), but in cellular assays it potently inhibited signaling through JAK1 and JAK3 with 5-100 fold selectivity over JAK2.