Fluorescent transgene expression in mouse embryos. (A) β-globin marks primitive erythroblasts in circulation within the yolk sac and embryo proper at 8.5 dpc. (B,C) The Flk1 promoter is used to drive membrane mCherry (B) and nuclear YFP (C) in endothelial cells of the developing embryo at 8.5 dpc. (D) A close-up image of vessels expressing YFP and mCherry in distinct subcellular localizations. (E) GFP expression driven by the cfms promoter in the neonatal eye (postnatal day 1), marking macrophages. (F) Expression of a double transgenic embryo at 7.0 dpc expressing GFP and red fluorescent protein (RFP) in cells derived from the visceral endoderm. (F, Reprinted from Hadjantonakis et al. 2008 with the permission from Elsevier © 2008.)