CreloxP

CreloxP refers to a widely used site-specific recombination system in molecular biology that enables controlled modification of DNA in cells and organisms. It combines the Cre recombinase enzyme with loxP DNA recognition sites to rearrange genetic sequences in a predictable manner.

The key components are loxP sites and the Cre recombinase. A loxP site is a 34-base pair sequence comprising two 13-base pair inverted repeats flanking an 8-base pair core. Cre recombinase, derived from bacteriophage P1, binds to loxP sites and catalyzes recombination between them.

Mechanistically, when two loxP sites flank a DNA segment (a floxed region), Cre can catalyze recombination between them. If the sites are aligned in the same orientation, the intervening DNA is excised; if the sites are in opposite orientations, the segment is inverted. More complex outcomes are possible when multiple loxP sites are present or when different lox variants are used.

Cre-loxP is widely employed for conditional gene modification, including tissue-specific knockouts, inducible gene activation, and lineage tracing in model organisms such as mice. Researchers drive Cre expression with tissue-restricted promoters or use inducible Cre variants (for example, Cre-ERT2 activated by tamoxifen) to control timing and location of recombination. LoxP-based strategies also enable activation of reporters or conditional rescue of gene function.

Several extensions exist, including alternative loxP variants (e.g., lox2272, lox5171) for sequential or combinatorial recombination and the use of additional recombinases (e.g., Flp) for intersectional genetic approaches. Experimental design requires attention to recombination efficiency, chromatin context, and potential unintended events, with appropriate controls.