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In molecular cloning and biology, a Knock-in (or Gene knock-in) refers to a genetic engineering method that involves the insertion of a protein coding cDNA sequence at a particular locus in an organism's chromosome. Typically, this is done in mice since the technology for this process is more refined, and because mouse embryonic stem cells are easily manipulated. The difference between knock-in technology and transgenic technology is that a knock-in involves a gene inserted into a specific locus, and is a "targeted" insertion.
A common use of knock-in technology is for the creation of disease models. It is a technique by which scientific investigators may study the function of the regulatory machinery (e.g. promoters) that governs the expression of the natural gene being replaced. This is accomplished by observing the new phenotype of the organism in question. The BACs and YACs are used in this case so that large fragments can be transferred.