ABSTRACT
In scientific research, it is often needed to knock in, knock out, knock down, or overexpress a specific gene in model organisms or specific types of cells to achieve precise regulation of experimental independent variables. In this case, various transgenic mice are required. The cyclization recombinase (Cre) can directly interact with different loxP (locus X over P1) DNA sequences without any cofactors to perform specific gene knock-in or knock-out at specific targets. Because of its advantages of simple action principles, high spatial specificity, and high reorganization efficiency, the Cre-loxP system is widely used in scientific research. Furthermore, the CreERT2 system (mutant of the fusion protein of Cre and estrogen receptor ligand binding domain) and the tetracycline (Tet)-on/off system, derived from the Cre-loxP system, have made the recombination of the target gene occur in temporal-specificity on the basis of spatial-specificity. This dual specificity of time and space is indispensable for research in specific directions such as fear memory and engram cells on the basis of reducing the impacts on experimental animals. Therefore, these derived systems have broad application prospects.