EpoR-HM is attenuated in its ability to support the development of Ter119-positive erythroid progenitor cells. (A) Marrow preparations from wt-EpoR mice were cultured in the presence of Epo at 1.6 U/mL, and Ter119-positive cell formation was assayed at 0, 14, 28, 42, and 56 hours (left). At 42 hours of culture, Epo concentration-dependent profiles for Ter119-positive cell development also were defined (right). (B) Bone marrow preparations from EpoR-HM or wt-EpoR mice were cultured in the presence of Epo at 0.1, 0.4, and 1.6 U/mL, and at 42 hours the Ter119-positive cell formation was assayed. Representative flow cytometric profiles are shown, and frequencies of Ter119-positive cells are indexed (as the percent of total events). The gate for positive events was set using unstained EPCs. (C) Differences in the abilities of EpoR-HM versus the wt-EpoR to support Ter119-positive EPC formation are summarized. Values are normalized means (less background levels for positive events in the absence of Epo stimulation) (n = 3 EpoR-HM and n = 3 wt-EpoR mice). Percents refer to positive events among totals.