Therefore, we examined the fluorescent expression of NT embryos at earlier time points. At 18 h postfusion 5% (n = 19) of NT embryos expressed fluorescence, and at 3 days postfusion, 8% (n = 51) of cleaved embryos expressed green fluorescence (unpublished data). But embryos with a mosaic expression were not detected at these stages. During culture, it is possible that some blastomeres may die and stop producing eGFP Another possibility is that eGFP gene expression, as well as that of other genes, is independent in individual blastomeres. In this case, only some blastomeres would express eGFP. Genetic mosaicism is not likely because these embryos are derived from a single donor.
Indeed, the animal from which the cells were derived has one integration site (Greg Bleck, personal communication) and all cells fluoresce (Fig. 1), which further suggests that she is not mosaic. Therefore, these results suggest that the gene expression pattern of NT embryos could be slightly different in each blastomere even though blastomeres were derived from a single cell and could be due to the site of integration. More work is needed to determine the exact control of gene expression. When the offspring of 402-2 is sufficiently expanded, then expression of this transgene during normal embryogenesis can be determined and directly compared with expression in in vitro-derived and NT-derived embryos.