Personal view of gene targeting

Gene targeting provides the means for creating strains of mice with mutations in virtually any gene.1 First, the desired mutation is introduced into a cloned copy of the chosen gene by standard recombinant DNA technology. The mutation is then transferred to the genome of a pluripotent mouse embryo-derived stem (ES) cell by means of homologous recombination between the exogenous, mutated DNA sequence and the cognate DNA sequence in the ES cell chromosome. By microinjection of ES cells containing the transferred mutation into blastocysts and by allowing the embryos to come to term in foster mothers, we can generate chimeric mice capable of transmitting the mutation to their offspring (germline chimeras). Finally, interbreeding of heterozygous siblings yields animals homozygous for the desired mutation. Figure 1 outlines the steps, from cultured ES cells to chimeric mouse, used to generate mice with targeted mutations.