In contrast, twin #2 who received from the same donor virtually the identical number of transplanted donor CD34+ cells and a slightly lower number of transplanted CD3+ T cells, received horse ATG with the conditioning regimen and experienced very rapid and complete donor T cell chimerism which was associated with both acute GVHD and extensive chronic GVHD (Fig. 3a–d). This evidence concerns the gene CD34 and acute graft versus host disease.