BLT xenograft systems are known to show a robust engraftment and early human immune reconstitution in NOD.Cg-PrkdcscidIL-2Rγctm1Wjl (NSG) mice (22, 23), but inopportunely, the variable early onset of xenograft GVHD (xeno-GVHD) in this BLT model is frequently lethal (24, 23). Here, CTSG is linked to graft versus host disease.