Fig. 8. PU.1 promoter expression in the early mouse embryo and ES cells. (A) GFP fluorescence in PU.1^G/+ heterozygote E3.5 blastocyst determined by confocal microscopy. (B) Immunodetection of the PU.1 protein in ES cells. (C) Confocal microscopy detection of GFP fluorescence in two optical sections of the same E6.5 PU.1^G/+ postimplantation embryo (left, tangential upper section; right, middle section). Arrow shows the area of the node that displays a marked concentration of GFP-positive cells in an as yet undefined structure. Images shown in A and C are representative of those from a series of 20-30 embryos from three litters. Scale bars: 20 µm in A,B; 80 µm in C.