CD4 and HIV-1 infection: In particular, the engraftment of human CD34+ fetal liver or umbilical cord-derived blood stem cells into highly immunodeficient NSG (NOD/Ltsz-scid/scidγcnull) mice allows reconstitution of a leukocyte pool that is human-derived, allowing chronic HIV-1 infection of the human CD4+ T cells (Denton and Garcia, 2011).