G1-HRD-luc activity during EPO treatment. (A) Serial imaging of G1-HRD-luc during EPO treatment. Mice were treated with EPO or saline for 5 consecutive days. Bioluminescent images were taken from mice treated with EPO (bottom panels) or saline (top panels) for 8 consecutive days. (B-D) Changes in G1-HRD-luc activity (B), hematocrit (C), and reticulocyte counts (D) in EPO-treated (▴) or saline-treated (⋄) spleens (n = 3 in each group) over a period of days. G1-HRD-luc activity on day 0 was set to 1 in each group. Error bars indicate SD. *P < .05 and **P < .01 compared with untreated control.