Figure 2: Generation of tissue/cell-specific knockout mice using the Cre-loxP system. In the Cre mouse line, the expression of Cre is under the control of a tissue/cell-specific promoter. The floxed target gene mouse line contains loxP sites (◀) flanking the region of the target gene to be deleted. When the two mouse lines are bred together, the Cre enzyme recognizes the loxP sites and deletes the intervening DNA sequence only in tissues/cells where the Cre is expressed. The target gene remains floxed and theoretically functional, in all other tissues.