The biological process by which genetic information is transferred or exchanged between regions of DNA of identical or very similar nucleotide sequence is known as homologous recombination. This natural process of genetic exchange appears to be ubiquitous and the study of its fundamental basis represents an important and exciting area of research in its own right. The potential use of homologous recombination to replace specified genes in both prokaryotic and eukaryotic organisms has resulted however in the investigation of this fundamental process with more applied purposes in Successful allele replacement promises not only to facilitate studies of gene function and regulation(134) but also to allow the engineering of genetic improvements and the elimination of undesirable traits from an organism without altering its genetic organisation or constitution.
Applying this technology to plants is highly desirable, but not simple. This is due to the fact that, like in animal cells, foreign DNA introduced by transformation integrates into the plant genome in an apparently random fashion, regardless of homology to endogenous This hurdle first seemed surmountable when gene targeting to restore a partial non-functional artificial gene was reported in plant protoplasts retransformed, by direct gene transfer, with homologous