INCB018424 is an orally bioavailable, potent and selective inhibitor of JAK 1 and JAK2, currently in clinical trials to treat myelofibrosis. In enzyme assays it inhibited JAK1 (IC50=3.2 nM) and JAK2 (4.1 nM), and had selectivity >100 fold for a wide range of other kinases, including JAK3 (428 nM). It acts by blocking the JAK/STAT pathway, and has IC50 of 280nM in cytokine-stimulated whole blood assays.