An enhancer trap transgenic approach was used to generate these mice. The longtk-cre:ERT2-SV40pA transgene was designed with the long form of the thymidine kinase minimal (TK) promoter driving a cre-ERT2 fusion gene (cre recombinase fused to a G400V/M543A/L544A triple mutation of the human estrogen receptor ligand binding domain). The SV40 polyadenylation signal follows the cre sequence. This transgene was injected into C57BL/6J embryos. Founder line 837 was established on a C57BL/6J genetic background.