The coding region was replaced with a construct encoding a tamoxifen dependent cre recombinase. The cre recombinase coding region was joined to sequence encoding the ligand binding domain of the human estrogen receptor with a glycine to arginine missense mutation at position 521. This substitution enables the domain to bind to tamoxifen while ablating the estradiol binding ability. Tamoxifen treatment permits the translocation of the cre hybrid protein to the nucleus.