Cre-loxP System

The Cre-loxP system is the standard tool for spatially and temporally restricted gene manipulation in the mouse. Cre (cyclization recombinase) is a 38-kDa tyrosine site-specific recombinase from bacteriophage P1, in the same family as Flp (flipase) and Dre. It recognizes loxP (locus of x-over, P1), a 34 bp sequence made of two 13 bp inverted palindromic repeats flanking an 8 bp core.

A single Cre enzyme recognizes two directly repeated loxP sites and excises the intervening DNA, which is described as “floxed”, yielding a circular excised product and an inactivated allele. Although excision is the predominant use, the orientation and location of the loxP sites determine whether Cre instead inverts or translocates the intervening DNA.

The system is popular because manipulation is simple and recombination is efficient without requiring additional cofactors. Refinements under development include Cre fused to fluorescent reporters, expression-optimized Cre, and split Cre, in which the N- and C-terminal halves are expressed from different promoters so that both must be active in the same cell.