Gene targeting: tapping the cellular telephone.

Gene targeting means that we now have the potential to generate mice of virtually any desired genotype. In the first instance, standards recombinant DNA technology is used to alter a cloned DNA sequence of a chosen locus; the modified potent stem cell derived from a mouse embryo, and homologous recombination between the exogenous and endogenous chromosomal sequence transfers the mutation to the genome. Microinjection of the stem cells containing the modified locus into mouse blastocysts is used to generate germ-line chimaeras. Finally, interbreeding of heterozygous siblings yields animals homozygous for the desired mutation.