We tested this in our T-cell-replete MHC-haploidentical HCT model by giving CD4+CD25+ donor Tregs 4 days prior to HCT; GVHD lethality was delayed, but ultimately these mice still developed severe and fatal GVHD, suggesting that donor Tregs, while necessary for GVHD prevention by PTCy, may not be sufficient to prevent GVHD after T-cell-replete MHC-haploidentical HCT (23). This evidence concerns the gene CD4 and graft versus host disease.