Loss of Gfi1 and Gfi1b blocks the release of blood cells from the yolk sac. (A) Fluorescence and bright field images of E9.5 HET/HET and KO/KO embryos and corresponding yolk sacs (2.5× objective). (B) FACS analysis of KIT/TIE2 expression among GFP⁺ cells of the indicated genotypes. Numbers indicate percentages.