Overexpression of epo expands erythroid populations. Capped epo mRNA was injected into 1-cell–stage zebrafish embryos and evaluated by whole-mount in situ hybridization. A clear increase in (A) scl⁺ and (B) beta e1-globin⁺ cell number in response to exogenous epo is evident in circulation at all stages examined, as well as in the kidney primordium beginning at 4.5 dpf. The arrowhead marks the enlarged vein lumen caused by excess cells in circulation. Lateral views, anterior to the left. (C) An expansion of c-myb⁺ and epor⁺ cells can be observed in the kidney primordium (boxed) at 7 dpf, dorsal views, anterior to the left.