Stat5^−/− embryos are severely anemic. (A) wt and Stat5^−/− E13.5 embryos (left) and newborn animals (right). (B) Ter-119⁺ erythroid cells (left) and fetal liver cellularity (right; data are presented as mean ± SD; n = 6) of wt versus Stat5^−/− fetal livers. (C) ELISA for Epo from serum of wt and knockout E16.5 embryos and newborns. Data are presented as means ± SD; n = 5. *P < .05; **P < .01.