Comparison of engraftment characteristics in various hematopoietic systems that differ in their donor and host HSC competitive capacity, assuming a model of relatively frequent stem cell cycling in nonirradiated systems. (A) Irradiated postnatal environment. Damaged microenvironment results in engraftment of a few donor HSC that reconstitute the recipient by oligoclonal expansion, allowed by the absence of host cell competition. There is an amplified “readout” of a few engrafted donor HSC. (B) Allogeneic IUHSCTx. Although a few donor HSC may engraft, they are at best equal, or more likely, at a competitive disadvantage to host HSC (mismatched stroma). Therefore, the engraftment is obscured by host hematopoiesis leading to a readout of minimal or microchimerism. (C) The syngeneic nonmyeloablated model. Engraftment through serial transplants or massive doses of donor cells results in multiple-donor HSC engrafted that can equally compete (genetically identical HSC/matched stroma) with host HSC. Engraftment readout is quantitatively reflective of No. Donor HSC/No. Host HSC. (D) Allogeneic IUHSCTx into a host with impaired HSC. Only a few donor HSC engraft, but oligoclonal expansion can occur despite stromal mismatch, due to reduced host HSC competition, leading to an amplified readout of donor HSC engraftment.