Cre-stimulated recombination at loxP-containing DNA sequences placed into the mammalian genome

The cre gene of coliphage P1 encodes a 38 kDa protein which efficiently promotes both intra- and intermolecular recombination at specific 34 bp sites called loxP. To demonstrate that the Cre protein can promote DNA recombination at loxP sites resident on a mammalian chromosome, a mouse cell line was constructed containing two directly repeated loxP sites flanking a 2.5 kb yeast DNA fragment and inserted between the SV40 promoter and the neo structural gene to disrupt expression of the neo gene. Expression of the cre gene in this cell line results in excision of the intervening yeast DNA and thus permits sufficient expression of the neo gene to allow cell growth in high concentrations of G418. Southern analysis indicated that Cre-mediated excision occurred at the loxP sites. In the absence of the cre gene such excisive events are quite rare. Cre-mediated recombination should thus be quite useful in effecting a variety of genomic rearrangements in eukaryotic cells.