Human SRCs are enriched within the CD34⁺CD38^– subset following expansion divisions. FACS-sorted BM CD34⁺CD38^– cells (4 × 10⁴) were cultured × 7 days with HUBECs + GM36SF. At day 7, the progeny of culture were collected, stained with anti-CD34–FITC and anti-CD38–PE, and flow cytometric analysis and cell sorting was performed. (A) Sterile FACS sorting of day-7 CD34⁺CD38^– and CD34^–CD38^– cell subsets was performed and each population was collected separately. NOD/SCID mice (N = 7 per group) received transplants of the collected cell subsets and human CD45⁺ cell engraftment was measured after 8 weeks. SRC activity was detected only within the day-7 CD34⁺CD38^– population, whereas SRC activity was not demonstrable within the day-7 CD34^–CD38^– subset. (B) Lineage distribution of engrafted human cells is shown within a representative mouse 8 weeks after transplantation with day-7 FACS-sorted CD34⁺CD38^– cells. (Bi) No huCD45⁺ cell engraftment is demonstrable within a representative mouse that received a transplant of day-7 CD34^–CD38^– cells. (Bii) huCD45⁺ cell engraftment is evident in a mouse that received a transplant of day-7 FACS-sorted CD34⁺CD38^– cells. CD34⁺ progenitor cell engraftment (Biii), CD19⁺ B-cell differentiation (Biv), and CD13⁺ myeloid differentiation (Bv) are also shown. PerCP indicates peridinin chlorophyll A protein; and Mu45, murine CD45.