Family circle

All the mouse embryos above belong to the same litter but half of them have inherited a transgene from their father (the green fluorescent adult mouse on the right) that causes them to glow yellowish green under fluorescent light. In these transgenic mice, the enhanced green fluorescent protein (EGFP) is expressed in all cells except hair and erythrocytes. The fact that there are equal numbers of fluorescent and non-fluorescent siblings indicates that the transgene is inherited in a Mendelian fashion. Mice such as these have many uses in tracking specific cell lineages during development in chimeric animals. In this case, they have been used to establish trophoblast stem (TS) cells, and studies tracking the fluorescent TS cells show that they are only incorporated into the placenta, not the embryo. For more details, see Hadjantonakis A-K et al.