JAK1 and chronic graft versus host disease: Ruxolitinib (Jakafi®) (Incyte Corporation, Wilmington, DE, USA) is a selective JAK1/2 inhibitor, was the first agent approved by the U.S. Food and Drug Administration (FDA) for the treatment of steroid-refractory acute GVHD (aGVHD) in 2019 and chronic GVHD (cGVHD) in 2021 [12].