A potent, ATP-competitive inhibitor of JAK2 that less effectively inhibits JAK3 (IC50s = 3 and 48 nM, respectively); inhibits JAK2 containing the V617F mutation (IC50 = 20 nM); reduces the growth of Ba/F3 pro-B-cells in SCID mice without affecting erythroid progenitors, reticulocytes, or platelets.