Posttransplantation increase in circulating progenitors is seen only with α4 integrin–deficient donor cells. (A) Transplantation experiments were performed by injecting α4-deficient (□) or α4-sufficient (+/+ or +/Δ; ▪) BM cells into normal recipients (2.5-5.0 million BM cells were transplanted into 4-7 mice/group), and (B) by injecting α4^+/+ cells into normal (▪) or α4-deficient (□) recipients. (One half million BM cells were transplanted to 5 mice/group.) A significant increase in circulating hematopoietic progenitor cells is seen only with α4^Δ/Δ donor cells and is maintained up to 29 weeks after transplantation; at that time, 2.9% ± 0.3% of BM cells and 3.0% ± 0.5% of WBCs were α4⁺ in mice given transplants (Figure S1B). No increase in circulating progenitors above control levels is seen when normal cells are transplanted into α4-deficient recipients. The asterisk indicates a significant difference between transplanting α4-deficient and α4-sufficient donor cells. Error bars indicate SEM.