CD4+ and CD8+ T cells were observed in blood starting at 20 weeks and continuing to 30 weeks post-transplantation, with initially higher CD4:CD8 ratios at 20 and 25 weeks, with subsequent decreases in CD4+ T cells and increases in CD8+ T cells coincident with the development of xeno-GVHD at 30 weeks. This evidence concerns the gene CD4 and graft versus host disease.