G1-HRD-luc expression in response to hypoxic exposure. (A) G1-HRD-luc mice were exposed to 6% oxygen for 4 hours and then returned to room air. Bioluminescent images were taken from 5 to 96 hours after hypoxic exposure began. The 0-hour image was taken just before exposure to hypoxia. Bioluminescent images overlaid with photographs of the lateral view are shown. (B) Average change in photon output from spleens exposed to hypoxia (▴,n = 4) and normoxia (⋄,n = 4). G1-HRD-luc activity in the spleen before the hypoxic exposure (0 hour) was set to 1. (C) Average change in the hematocrit of hypoxia-exposed (▴, n = 6) and normoxiaexposed (⋄,n = 4) mice. (D) EPO concentrations in the plasma before (0 hours) and 5 hours after hypoxic exposure (0 hours, n = 3; 5 hours, n = 4). Error bars indicate SD. *P < .05 compared with untreated control.