Targetted Gene Transfer - In targetted gene transfer, the trans gene integrates at the precise site where its allele is present in the genome. This is achieved by a process of homologous recombination by which the trans gene replaces its allele in the genome.

Homologous recombination may be defined as the exchange of identical segments between two DNA molecules that have identical or almost identical sequences (the differences in their sequences are limited to allelic differences). In yeast, gene integration occurs ordinarily by homologous recombination.

The approaches available for the identification of homologous recombinations are as follows:

(1) When inactivation or activation of the test gene occurs due to homologous recombination, but not by random integration, and produces a selectable phenotype, such cells can be scored and selected for.

(2) Alternatively, a large number of transfected cells/clones may be screened, often using PCR amplification, to identify those having homologous recombination.

Targetted gene transfer has the following two chief applications.

(1) It is the ideal approach for gene therapy. Further,

(2) it permits an analysis of the functions of or the effects produced by a gene cloned and/or subjected to site-directed mutagenesis in a complete animal.