Abrogation of STAT5 blocks epo-induced polycythemia.o-dianisidine expression in (A) uninjected, (B) STAT5 MO–injected, (C) epo mRNA–injected, and (D) STAT5 MO:epo mRNA–injected embryos at 36 hpf, 2.5 dpf, and 4 dpf. Embryos injected with STAT5 MO show a slight decrease in erythropoiesis beginning at 36 hpf, and by 4 dpf there are very few erythrocytes in circulation. Injection of epo mRNA alone causes a progressive increase in the number of o-dianisidine⁺ cells in circulation. To demonstrate that STAT5 is required for signaling through the EpoR, STAT5 MO was coinjected with epo mRNA. Loss of STAT5 effectively blocks the expansion of erythrocytes by epo overexpression as shown by a decrease in o-dianisidine⁺ cell numbers. Lateral views, anterior to the left.