Fig. 1.

Cre recombinase in genetics. A Cre recombinase recognizes two repeated 34 bp loxP sites in DNA, then excises loxP-flanked sequences, leaving a single loxP site. More complex rearrangements can be made with changes to the orientation and position of the recognition sites. B To excise sequences in vivo, a mouse expressing Cre recombinase under the control of a tissue or temporal-specific driver is crossed with a mouse carrying LoxP sites that flank a gene or genes of interest. Progeny carrying both the loxP-flanked gene and the Cre recombinase will show deletion in the tissue of interest at the stage when the driver is expressed