Definitive hematopoiesis in the yolk sac at E9.5. (A) Quantitative analysis of definitive hematopoietic progenitor cells as analyzed by CFU/YS of F2 embryos at E9.5. Data represent mean (± SD) of at least 7 embryos analyzed for each genotype. There was no significant difference among genotypes. (B) Immunoblot analysis of Rac1 protein expression level of individual hematopoietic progenitor colonies from the yolk sac of TGRac1^Flox/null mice (lanes 3-9) and control mice (lanes 1-2) at E9.5. β-Actin is shown as a loading control. Note intensity of Rac1 band in lane 2 versus lane 1, representing expression of Rac1 in the heterozygous versus WT genotype. These results are representative of 3 independent experiments with similar results.