Ruxolitinib is a JAK1 and JAK2 inhibitor with potent anti-inflammatory properties and excellent safety profile, that is approved for the treatment of myelofibrosis [12, 13] and polycythemia vera [14, 50, 51]; remarkably, ruxolitinib proved to be efficacious in conditions characterized by exaggerated release of inflammatory cytokines and activation of immunocompetent cells, such as the hemophagocytic lymphohistiocytosis [15] and the graft-versus-host disease in recipients of allogeneic hematopoietic stem cell transplantation [16]. The gene discussed is JAK2; the disease is hemophagocytic syndrome.