The murine immune system is not necessarily identical to it human counterpart, which has led to the construction of humanized mice. The current study analysed whether or not a human immune system contained within the non-obese diabetic (NOD)-Rag1null-γ chainnull (NRG) mouse model was an accurate representation of the original stem cell donor and if multiple mice constructed from the same donor were similar to one another. To that end, lightly irradiated NRG mice were injected intrahepatically on day 1 of life with purified cord blood-derived CD34+ stem and progenitor cells. Multiple mice were constructed from each cord blood donor. Mice were analysed quarterly for changes in the immune system, and followed for periods up to 12 months post-transplant. Mice from the same donor were compared directly with each other as well as with the original donor. Analyses were performed for immune reconstitution, including flow cytometry, T cell receptor (TCR) and B cell receptor (BCR) spectratyping. It was observed that NRG mice could be ‘humanized’ long-term using cord blood stem cells, and that animals constructed from the same cord blood donor were nearly identical to one another, but quite different from the original stem cell donor immune system.