loxP

loxP is a 34-base-pair DNA sequence that functions as the recognition site for the Cre recombinase, forming the cornerstone of the Cre-loxP system used to perform site-specific recombination in living cells. This system enables controlled manipulation of DNA segments in a variety of organisms, including mice, plants, and cultured cells.

The loxP site consists of two 13-base-pair inverted repeats separated by an 8-base-pair spacer. The canonical sequence is 5'-ATAACTTCGTATA-GCATACATTATACGAAGTTAT-3'. The orientation of two loxP sites dictates the outcome of recombination events.

In the presence of Cre recombinase, recombination occurs between two loxP sites. If the sites are in the same orientation on a single DNA molecule, the intervening sequence is excised as a circular DNA and the ends are joined, resulting in deletion. If the sites are in inverted orientation, the sequence between them is inverted. When loxP sites lie on different DNA molecules, recombination can join the molecules.

History and usage: The loxP site was derived from bacteriophage P1 and paired with Cre recombinase to form the Cre-loxP system, developed and popularized in the late 1980s and 1990s. It is widely used for creating tissue-specific or inducible genetic modifications, conditional knockouts, and lineage tracing in model organisms.

Variants: To enable more complex genetic strategies, several lox site variants (such as lox2272, lox5171) have been engineered to recombine preferentially with identical variants, allowing multiple rounds of recombination without cross-reactivity. The system is often coupled with inducible Cre fusion proteins to control when recombination occurs.