JAK2 and graft versus host disease: Studies on the JAK1/JAK2 inhibitor Ruxolitinib also reported an increased susceptibility for viral re-activation of Hepatitis-B and varicella zoster virus in treated patients with myeloproliferative neoplasm and polycythemia vera, but also a modestly higher incidence of infection and reactivated CMV infection in patients with steroid-refractory GVHD (198, 233, 234).