In biomedical research, gene-targeted mice are animals used to study the physiological functions of a given gene in the intact animal by deleting the gene or functional domain(s) of the gene (knockout) (Fig. 2A). Gene targeting is also used to generate a "humanized" mouse - one expressing a given human gene, with or without mutations, under the control of the mouse regulatory elements (knock-in) (Fig. 2B). For both purposes, a sequence replacement vector containing a homologous DNA sequence is used to replace the chromosomal sequence in mice.
Mice generated with a knockout strategy represent loss-of-function mutations, which usually proves whether or not the missing gene is important for a given physiological function. However, mice generated with a knock-in strategy can represent a gain-of-function, either physiologically or pathophysiologically, if the gene has a normal sequence or dominant negative mutations, respectively.
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