The mechanism of human stem cell expansion ex vivo is not fully understood. Furthermore, little is known about the mechanisms of human stem cell homing/repopulation and the role that differentiating progenitor cells may play in these processes. We report that 2- to 3-day in vitro cytokine stimulation of human cord blood CD34+-enriched cells induces the production of short-term repopulating, cycling G1 CD34+/CD38+ cells with increased matrix metalloproteinase (MMP)-9 secretion as well as increased migration capacity to the chemokine stromal cell-derived factor-1 (SDF-1) and homing to the bone marrow of irradiated nonobese diabetic severe/combined immunodeficiency (NOD/SCID) mice. These cycling G1 cells enhance SDF-1-mediated in vitro migration and in vivo homing of quiescent GO CD34+ cells, which is partially abrogated after inhibition of MMP-2/-9 activity. Moreover, the engraftment potential of quiescent GO SCID repopulating cells (SRCs) is also increased by the cycling G1 CD34+/CD38+ cells. This effect is significantly abrogated after incubation of cycling G1 cells with a neutralizing anti-CXCR4 antibody. Our data suggest synergistic interactions between accessory cycling G1 CD34+/CD38+ committed progenitor cells and quiescent, primitive G0 CD34+/CD38 -low SRC/stem cells, the former increasing the motility and engraftment potential of the latter, partly via secretion of MMP-9.