As well as mutating or knocking out specific genes, gene targeting allows the introduction of novel pieces of DNA into a specific chromosomal location (this is often termed a "knock-in"). This allows researchers to examine the function of a gene in a variety of ways. For example, it is possible to examine where in the animal the gene is normally expressed by insertion (knock-in) of a fluorescent protein (such as green fluorescent protein, GFP) into the gene so that the cells expressing the gene begin to glow. In addition to changing single genes it is also possible to remove or alter large pieces of chromosomes.
Technologies also now exist that allow genes to be removed not just in a whole animal, as described above, but in a subset of cells or in a particular tissue. This can be achieved by modifying the vector to include target sites (termed loxP sites) for an enzyme called Cre recombinase. When the Cre enzyme is present in a mouse cell in which the target gene is surrounded by loxP sites, it will cut this gene out of the chromosome. This allows the function of this gene, which may be required for the mouse to normally develop, to be analyzed in a particular cell type or tissue where only the Cre recombinase is expressed.
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