The indicated number of donor-type (CD45.2⁺) FLK2^–Sca-1⁺Lineage^–c-kit⁺ cells from young adult bone marrow, old adult bone marrow, reconstituted bone marrow, and day-7 cyclophosphanide/G-CSF–mobilized splenocytes were transplanted intravenously into lethally irradiated recipients (CD45.1⁺) along with recipient-type 200 000 whole bone marrow cells. Recipients were considered engrafted by donor cells if any CD45.2⁺ cells were detected in their peripheral blood (above background: > 0.3 of myeloid cells or > 0.1-0.15 of lymphoid cells). Mice were considered long-term multilineage reconstituted if donor-type myeloid, B, and T cells were present for at least 16 weeks after reconstitution. The frequency of cells that gave long-term multilineage reconstitution (HSCs) was calculated based on limitdilution Poisson statistics.²¹  NA indicates not applicable because no HSC activity was detected; CNBC, could not be calculated because all mice were LTMR.