In living cells,exploiting homologous recombination between sequences both from genomic DNA and extracellular DNA to accomplish site-modification in a certain gene on chromosome,is a method called gene targeting.Although the molecular mechanism of DNA homologous recombination is far from being elucidated, it is a common biological phenomenon.It is true that in cells there is an enzyme system that plays the role to make recombination of homologous DNAs, which is the theoretical foundation of gene targeting. The technology of gene targeting has been fully developed. The key step for operation is to construct a recombinant carrier including one or two genes for selection and to transfer it into the nucleus effectively. The targeted cells can be inherited stably. Gene targeting has promising prospects in production of new strains of living things, in clinical usage and in theoretical research of some complicated biological phenomena such as the molecular mechanism of development.