Cre/lox is usually used to make knockout alleles, but it can also be used to activate gene expression. The proper insertion of a loxP-flanked “stop” sequence (transcriptional termination element) between the promoter and transgene coding sequence blocks the expression of the gene.

Cre, or cyclic recombinase, is one such enzyme. Site-specific recombination is, thus, the enzyme-mediated cleavage and ligation of two defined deoxynucleotide sequences. A number of conserved site-specific recombination systems have been described in both prokaryotic and eukaryotic organisms.

Conditional knockout mice (also known as Flox mice) are mice whose target genes contain paired loxp sites. After mating them with Cre mice, target genes can be knocked out in specific tissues or cells.

The Discovery of Cre Mice Cre is an enzyme that recognizes specific DNA sequences (loxP) and causes them to rearrange. This system allowed scientists to selectively control the expression of genes in certain cells or tissues, leading to the development of genetically engineered mice with specific gene mutations.