recombinases

Cyclization recombination enzyme (Cre) is a member of the extensive family of recombinases and recognizes a 34 bp sequence motif from PI bacteriophage referred to as LoxP. The Cre enzyme works to cleanly excise an intervening DNA fragment that is flanked by two LoxP sites. The LoxP sites must be present in the same orientation. The excised segment is later degraded to leave only a single LoxP site copy in the starting target molecule.