JAK1 and graft versus host disease: The Janus kinase (JAK1 and JAK2) inhibitor ruxolitinib reportedly promotes Treg differentiation and ameliorates GVHD.57–59 Accordingly, treatment with ruxolitinib increased the frequency of epithelial Treg cells following allo-BMT (Figs. 1i, S1h, i), reduced acute GVHD morbidity (Fig. S1j), and enhanced ex vivo organoid regeneration.