Loss of EpoR decreases primitive and blocks definitive erythropoiesis. beta e1-globin expression in (A) uninjected, (B) epor MO–injected, (C) epo mRNA–injected, and (D) epor MO:epo mRNA–injected embryos at 36 hpf, 2.5 dpf, and 4 dpf. Embryos injected with epor MO show a slight decrease in erythropoiesis beginning at 36 hpf, and by 4 dpf there is a complete absence of circulating erythrocytes. Injection of epo mRNA alone causes a progressive increase in the number of beta e1-globin⁺ cells in circulation throughout ontogeny. To demonstrate that epo and the epor are a functional ligand-receptor pair, epor MO was coinjected with epo mRNA. Loss of EpoR effectively blocks the expansion of erythrocytes by epo overexpression as shown by a decrease in beta e1-globin⁺ cell numbers. Lateral views, anterior to the left.