In agreement with previous studies, the lack of mouse MHC class I molecules in B2m-NOG mice, due to β2-microglobulin gene deletion (B2m), allowed preferential engraftment of CD4 T cells over CD8 T cells and delayed the onset of xenogeneic graft versus host disease (GVHD) [1,19]. The gene discussed is CD4; the disease is graft versus host disease.