T helper 17 (Th17) cells, a novel subset of CD4+ T cells, are known to secrete IL-17A, IL-17F, IL-22, GM-CSF, and IFN-γ, which are involved in inflammation, autoimmune diseases, and graft-versus-host disease (GVHD) (67–72). This evidence concerns the gene IL17F and graft versus host disease.