Some biotechnology applications of site-specific recombination. (a) Gene activation (by deletion of a transcription stop signal). (b) Gene insertion. (c) Allele replacement, using two variants of the same recombination target site at each end of the gene (or other sequence) to be replaced. In each case, the site-specific recombination target site is engineered into the chromosome or into the introduced DNA, or both, with the orientation indicated by the arrows.