Gene targeting technology has been most commonly used to produce null mutations or gene knockouts. However, it can also be used to selectively modify endogenous mouse genes down to the level of introducing single nucleotide changes, producing what are known as knock-in mice. In contrast to pronuclear injection, in which multiple copies of a transgene are inserted randomly, in gene targeting, the normal mouse gene is modified in its normal chromosomal position. Thus, although in pronuclear injection a transgene is typically overexpressed and often misexpressed spatially and temporally because of its coupling to a heterologous promoter, a gene-targeted allele is expressed at normal levels and in a normal temporal and spatial expression pattern.
