Transgenic Animals

Targeted Gene Replacement and "Knockouts"

A gene that is injected into a fertilized egg is generally integrated randomly into the host genome. This means that scientists originally had no control over where in the host genome the foreign gene would land, nor could they control the number of copies of the gene that would be integrated. Where and how many copies of a gene are inserted can profoundly affect its function, so scientists looked for ways to make more precise insertions.

In the late 1980s Mario Capecchi and colleagues pioneered a method to target the inserted gene to a desired position in the genome. These researchers took advantage of an observation that, on rare occasions, an injected, mutated copy of a gene lines up precisely with the original form of the gene in the mouse genome. By a process called homologous recombination, the aligned DNA segments are cut and rejoined to each other. The result is a precise stitching of the introduced DNA into the targeted gene in the mouse genome. This means that scientists found they could make minor modifications to a gene before injecting it and, by homologous recombination, or "gene targeting," replace the natural gene with this transgenic version. For commercial applications, the transgene is often a gene coding for a functional human protein, which is then mass-produced in the host organism and isolated. For research purposes, it is often more useful to insert a mutated, nonfunctional version of the gene, to see what happens when the normal, functional version is missing. Creating such "knockout" organisms is a key tool used for studying genes that control development.