Cre-loxP system

A single Cre recombinase recognizes two loxP site (34 bp), then the Cre excises the loxP flanked (floxed) DNA, thus creating two types of DNA with circular, excised and inactivated gene.

Cre (Cre recombinase)

One of the tyrosine site specific recombinase (38 kDa)
Cre recombinase is expressed by a promoter that specifically targets the cell or tissue of interest.