[[Cre recombinase|Cre]] belongs to a family of [[enzymes]] called [[recombinases]]. Cre ('''c'''ausesyclic '''re'''combinationcombinase) is able to recombine specific sequences of DNA without the need for [[Cofactor (biochemistry)|cofactors]]. Cre recombinase recognizes a 34 base pair DNA sequence called loxP. Upon encountering two separate loxP sites flanking a target nucleotide sequence along a linear DNA fragment, Cre deletes this intervening sequence. Tissue-specific gene knockout is achieved by the excision of a lox-P flanked (''floxed'') critical region of a gene after Cre is expressed in the tissue of interest. Depending on the orientation of target sites with respect to one another, Cre will excise, exchange, integrate, or invert DNA sequences. The excision reaction is effectively irreversible, and has been most successfully carried out in the mouse. Upon the excision of a particular region of DNA by the CreloxP system, normal gene expression is considerably compromised or eliminated.
