JAK1 and chronic graft versus host disease: Janus kinase (JAK1/2) inhibition is now regarded as a therapeutic strategy for severe acute and chronic GVHD; it seems to be effective both as acute GVHD prophylaxis, in the treatment of steroid-refractory GVHD, and in salvage therapy for chronic GVHD [4,5,6,7,8,9,10].