Figure 1.

Deletion of loxP flanked sequences by the Cre recombinase. A fragment of the NR1 (Grin1) gene is shown as a line with solid black rectangles representing exons. The numbers above the rectangles correspond to exons. The two triangles represent the loxP sequences introduced in introns, placed in the same orientation and flanking exons 11–18 (Niewoehner et al., 2007). The Cre recombinase has a nuclear localization signal and is normally shuttled to the nucleus after translation, where it catalyzes a deletion of the gene fragment flanked by the loxP sites. Thus, gene inactivation will occur soon after the gene promoter driving Cre expression becomes active, typically around the 13th day of embryonic development (Parlato et al., 2006). The CreERT2 is a fusion protein of the Cre and a modified ligand binding domain of the estrogen receptor (ERT2). The modification prevents binding of endogenous estrogens but allows binding of tamoxifen, a synthetic steroid (represented by a circle with a “T”). Additionally, the presence of the ERT2 domain enables interaction with the mechanisms normally responsible for keeping the estrogen receptor in the cytosol, in particular interaction with Hsp90. Binding of tamoxifen to the ERT2 releases its interaction with the cytosolic proteins and permits shuttling to the nucleus, where it catalyzes the deletion of the gene fragment flanked by loxP sequences.