Cre recombinase: the universal reagent for genome tailoring.

In September 1998 the workshop for “Conditional Genome Alterations” in Cold Spring Harbor had a simple but important take-home message: “Cre works” (Rossant and McMahon, 1999). After several years of careful investigations, geneticists are now certain that the Cre recombinase of the P1 bacteriophage efficiently catalyzes recombination between two of its consensus 34 base pair DNA recognition sites (loxP sites) in any cellular environment and on any kind of DNA. Obviously mouse geneticists, including myself, are among those who are most excited about this finding. The reason is obvious: When we acquired the Cre/loxP recombinase system as a tool, we reached the level of sophistication of “no more practical limitation” in tailoring the mouse genome. The well-established practice of transgenesis allows the addition of functional DNA pieces into the mouse genome. Furthermore, with the advent of mouse embryonic stem (ES) cells and homologous recombinationbased gene targeting, we are capable of removing any existing gene from the genome. By adding the Cre recombinase system, we can excise or invert loxP-flanked DNA segments or create intermolecular recombination between different DNA molecules. By combining all of these tools, we are capable of creating any desired modification of the mouse genome, from introducing specific point mutations to large site-specific chromosomal aberrations. Furthermore, using the recombinase activity as a genetic activation or inactivation switch, conditional transgenesis or conditional knock-outs became available. The strategies are designed; the proof of principles are given. Many of these strategies utilize a common set of reagents, namely transgenic mouse lines that express the Cre recombinase in a tissue-specific manner and with high fidelity with respect to the recombinase activity. The bottleneck for taking full advantage of these powerful technologies seems to be the realization of this universal set of reagents, the Cre transgenic lines. The purpose of this introductory review in this special issue of Genesis is to summarize the recent status of mouse genetic strategies that apply the Cre recombinase system. BASIC PROPERTIES OF CRE RECOMBINASE