Whole-mount analysis of transgene expression driven by the -950 EKLF promoter during embryogenesis. Expression is observed in the circulating primitive erythroid cells in the yolk sac of a transgenic d10.5 embryo (A) and in the definitive erythroid cells in the fetal liver of a transgenic d12.5 embryo (B), but not in a nontransgenic littermate (C). Original magnification, × 50 (A); and × 20 (B-C).