A knock-in (or gene knock-in) refers to a genetic engineering method that involves the one-for-one substitution of DNA sequence information in a genetic locus or the insertion of sequence information not found within the locus . Typically, this is done in mice since the technology for this process is more refined and there is a high degree of shared sequence complexity between mice and humans . The difference between knock-in technology and traditional transgenic techniques is that a knock-in involves a gene inserted into a specific locus, and is thus 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.
More recent developments in knock-in technique have allowed for pigs to have a gene for green fluorescent protein inserted with a CRISPR-Cas9 system, which allows for much more accurate and successful gene insertions . The speed of CRISPR-Cas9-mediated gene knock-in also allows for biallelic modifications to some genes to be generated and the phenotype in mice observed in a single generation, an unprecedented timeframe .
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