While the pharmacologic blockade of NADPH oxidase complex to reduce the production of ROS after transplantation ameliorated clinical GVHD and survival (22), the deficiency of Indoleamine 2,3-dioxygenase 1 (IDO1), scavengers for ROS, in donor cells exacerbated GVHD. Here, FMO5 is linked to graft versus host disease.