Jakafi (ruxolitinib) is a JAK inhibitor used to treat myelofibrosis (MF), polycythemia vera, and graft versus host disease (GVHD). It works by blocking Janus-associated kinases (JAK1 and JAK2), which are overactive in blood cancers and contribute to abnormal blood cell production. By inhibiting these kinases, Jakafi helps slow the production of unhealthy blood cells and reduces signals that drive GVHD.