Long-term competitive BM reconstitution assay. (A) LSK/EPCR⁺ or LSK/EPCR⁻ cells (2 × 10³ cells/recipient) were transplanted into lethally irradiated recipient mice together with BM-derived unfractionated competitor cells (2 × 10⁵ cells/recipient). The chimerism of donor-derived cells in the peripheral blood of recipient mice was significantly higher after transplantation of LSK/EPCR⁺ cells (); LSK/EPCR⁻ cells failed to reconstitute the BM of recipients (). Error bars represent SD (n = 3). **P < .01. (B) Engrafted BM was analyzed 4 months after transplantation for donor-derived LSK fraction, myeloid, lymphoid, and erythroid lineages (top panels). Bottom panels show the data obtained from untransplanted control mice. Representative FACS patterns are shown with data indicating percentage frequency of LSK or population in each quadrant. Data represent mean ± SD (n = 3). (C) Unfractionated BM mononuclear cells were obtained from primary recipients 3 months after transplantation and were transplanted (1 × 10⁶ cells/recipient) into a second set of lethally irradiated recipient mice. Primary donor–derived cells in LSK/EPCR⁺ recipient mice exhibited BM reconstitution activity with substantial chimerism of peripheral blood cells (), whereas no such activity was observed when cells from LSK/EPCR⁻ recipient mice were transplanted (). Error bars represent SD (n = 5).