A selective JAK2 inhibitor that blocks signaling (IC50 = 58 nM at 5 mM ATP) and proliferation of JAK2-expressing cell lines (GI50 = 60 nM); displays good pharmacokinetics in vivo, inhibits STAT3 and STAT5 activation in vivo, and suppresses tumorigenesis in mouse xenografts harboring constitutive STAT3 activity.