This transgene expresses a fusion protein consisting of Cre recombinase joined to the ligand-binding domain of a mouse estrogen receptor modified to bind to 4-hydroxytamoxifen, but not to endogenous estrogen. The CAG promoter, containing a chicken beta actin promoter/enhancer coupled with the cytomegalovirus immediate-early (CMV-IE) enhancer, drives high levels of expression in most tissues. In the presence of tamoxifen, the fusion protein is transported into the nucleus, where cre can excise loxP-flanked segments from conditionally modified genes. (J:76130)