A cDNA fragment encoding a fusion protein comprised of cre recombinase and the ligand binding domain of Esr1 was inserted at the endogenous locus via homologous recombination. The initiator methionine was placed in an optimal translational context, allowing for near ubiquitous expression via the endogenous promoter. Activation of cre recombinase activity is dependent on the administration of 4-HO-tamoxifen.