Cell surface phosphatidylserine levels are increased among EpoR-HM EPCs. (A) Bone marrow preparations from EpoR-HM and wt-EpoR mice were cultured in the presence of Epo at 0.1, 0.4, or 1.6 U/mL. At 42 hours of culture, frequencies of FITC–annexin V-positive (and -negative) cells were determined and are indicated as percents of total events. The gate for positive cells was set using unstained cells. Primary data are shown for representative EpoR-HM and wt-EpoR EPCs. (B) Also shown is a representative annexin V versus Ter119 flow cytometry profile for EpoR-HM EPCs (left) versus wt-EpoR EPCs (right) cultured in Epo at 0.4 U/mL (mid-range dose). (C) Graphed are direct frequencies of annexin V–positive and Ter119-positive cells formed from EpoR-HM (n = 3) and wt-EpoR (n = 3) mice at increasing doses of Epo (means ± SD).