JAK2 and graft versus host disease: These alloreactive lymphocytes, however, often also target the recipients’ tissues, inducing an inflammatory syndrome known as graft-versus-host disease (GvHD).1 GvHD is one of the major contributors to the high costs, the high morbidity, and the 10–30% transplantation-related mortality of allogeneic HCT, in addition to relapse of primary disease, organ failure, and infections.2–5 Treatment for GvHD consists of corticosteroids (prednisolone) and other immunosuppressants, such as the macrolide sirolimus, calcineurin inhibitors, and Janus-kinase-2 inhibitors such as ruxolitinib.