Nuclear GFP reporter for EryP. (A) Histone H2B-GFP expression within the yolk sac “blood islands” of an E8.5 ε-globin-H2B-GFP transgenic embryo. (B) GFP(+) cells within the yolk sac vasculature of an E9.5 ε-globin-H2B-GFP transgenic embryo. (C) Nuclear expression of H2B-GFP fusion protein permits identification of mitotic figures and actively dividing cells. Blue represents 4,6-diamidino-2-phenylindole. Embryos were imaged as in Isern et al.^12,39