We can. Mario R. Capecchi (US), Oliver Smithies (US), and Martin J. Evans (UK) share their prize for gene targeting in mice: homologous recombination can be applied to inactivate specific genes in embryonic cells. The accuracy is perfect. The methods are so far used for mice only but they always teach us things about life and aging in general.
The same gentlemen have received the 2001 Lasker Award so you can read an article about that prize.