Cre-recombinase is an enzyme that catalyzes recombination between two loxP sites. The LoxP sites are specific 34-base pair-long DNA sequences consisting of an 8-bp core sequence, where recombination takes place, and two flanking 13-bp inverted repeats. ¬†The Cre-lox system is a tool for generating knockouts, conditional knockouts, and reporter strains in GM mice. The Cre-lox mechanism was discovered in P1 bacteriophage as part of this organism’s normal viral live cycle. As the cre gene and loxP sites are not normally present in the mouse genome, their insertion by transgenic technology facilitates deletions, inversions, or chromosomal translocations. Typically, cre and loxP strains are developed separately and crossed to produce a Cre-lox. The majority of cre and loxP strains being developed fall into one of the following categories:

Cre-expressing strains

Express Cre-recombinase under the control of a widespread (general) or tissue-specific (conditional) promoter. They are used to produce general or conditional knockouts respectively. Cre-expressing lines are generated by micro injection of a transgene with a protein of choise, or by knock-in the Cre-recombinse in the desired locus of the mouse genome.

Inducible Cre strains

Contain a transgene that expresses a modified form of Cre recombinase that is non-functional until an inducing agent (such as doxycycline, tetracycline, RU486, or tamoxifen) is administered at a desired time point in embryonic development or adult life

LoxP-flanked (floxed) strains

Contain loxP sites flanking (on each side of) a critical portion of a target gene or genomic region of interest

Cre reporter strains

Contain loxP sites in combination with visible (fluorescent or lacZ) marker proteins used to trace Cre recombination sucess and/or alterations in gene expression.