To solve the problem of embryonic lethality in conventional gene knockouts, site-specific recombinase (SSR) systems (Cre-loxP, Flp-FRT, and PhiC31) have been used for tissue-specific gene knockout. With the combination of an SSR system and inducible gene expression systems (tetracycline and tamoxifen), stage-specific knockout and transgenic expression can be achieved. The application of this "SSR+inducible" conditional tool to genomic manipulation can be extended in various ways. Alternatives to conditional gene targeting, such as conditional gene trapping, multipurpose conditional alleles, and conditional gene silencing, have been developed. SSR systems can also be used to construct precise disease models with point mutations and chromosomal abnormalities. With these exciting achievements, we are moving towards a new era in which the whole genome can be manipulated as we wish.

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