Recent variants of these strains lacking MHC class I molecules (B2m) show increased CD4 to CD8 ratios and a longer time window before onset of xenogeneic graft versus host disease (GVHD) and represent improved strains for the study of human immune responses (King et al., 2009a; Morillon et al., 2020b). The gene discussed is CD4; the disease is graft versus host disease.