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The technique of homologous recombination pioneered by Capecchi and Smithies was initially done in mammalian cell culture, and proved to be a powerful tool in that system; however, the real impact of the work was only felt when Evans learned how to isolate embryonic stem cells from mice. Capecchi and Smithies went on to perform the homologous recombination procedure on these cells, which were extracted from early mouse embryos and could be easily grown in Petri dishes. When placed into other mouse embryos, they merged with them, creating chimaeras. In mice where the transformed cells populated the sperm-producing or egg-producing cells of the gonads, the progeny were transgenic.