Ruxolitinib is a selective inhibitor of the Janus kinases (JAKs) JAK1 and JAK2 (IC50 values for JAK1 and JAK2 are 3.3 nmol and 2.8 nmol, respectively). These enzymes mediate the signaling of a number of cytokines and growth factors that are important for hematopoiesis and the immune system.