Competitive repopulation assay to determine the relative stem cell content of ex vivo–expanded BM versus fresh nonexpanded marrow. C57 donor BM cells were transduced with the HaMDR1 vector. HW80 donor BM cells were transduced with the HaDHFR vector. Cells were expanded for 12 days in culture and then combined at the indicated hind limb volumes. 0.005 hind limb volumes of expanded cells were competed against 0.25 hind limb volumes of fresh competing marrow. MDR1 expanded cells competed effectively against fresh HW80 marrow despite the unfavorable donor volume ratio (left). DHFR-expanded marrow was completely outcompeted by fresh C57 marrow (middle). When MDR1 (C57) cells were competed against DHFR (HW80) cells at equal hind limb volumes, mice reconstituted solely with MDR1 marrow, indicating a much greater stem cell content (right; mouse identification numbers are 5, 6, 7, 8, 9, and 10).