EPCR expression accurately identifies HSC activity. (A) Flow cytometry of total bone marrow stained with both Sca-1 and EPCR antibodies showing that EPCR identifies a distinct subfraction of Sca-1^hi cells that constitutes approximately 10% of the Sca-1^hi population. Black, green, and red text indicates the percentage of total cells found to be Sca-1^hi, Sca-1^hi EPCR^–, and Sca-1^hi EPCR⁺, respectively (left panel). Right panel indicates level of engraftment 4 months after transplantation of 100 cells sorted using the gates shown in the left panel in competition with 200 000 unfractionated bone marrow competitor cells. EPCR expression clearly fractionates all stem cell activity from the Sca-1^hi population. (B) Gating logic used to isolate the 500 SP cells, either positive or negative for EPCR expression, that were transplanted in competitive repopulation experiments against 200 000 unfractionated bone marrow competitor cells (left panels). Engraftment of cells sorted 4 months after transplantation showing little engraftment activity by SP cells lacking EPCR (right panels). (C) Gating logic used to isolate 500 EPCR-expressing cells exhibiting either SP or MP dye-efflux phenotype (left panel). Repopulation of sorted cells 4 months after transplantation in competition with 200 000 unfractionated bone marrow cells showing significant engraftment by EPCR-expressing MP cells (right panel). Error bars represent standard deviation.