Hanash et al. [16] reported that IL-22 plays protective roles by decreasing tissue damage and mortality in graft versus host disease (GVHD); however, others have reported that IL-22 leads to acute GVHD by aggregating effector T cells and reducing the regulatory T cells (Treg), and that IL-22 deficiency attenuates murine acute GVHD mortality. The gene discussed is IL22; the disease is acute graft versus host disease.