chimaera• A quite different way of creating a chimaera is to fuse two early mouseembryos.• The embryo is thus a chimaera, a mosaic of two different kinds of cells.• Two out of 160 G418-resistantD3 clones were found to have the targetedmutation and clone 515 gave rise to germ-line chimaeras.• Germ-line chimaeras were obtained from both clones.