An enhancer trap transgenic approach was used to generate these mice. The UAS-tk/hsp-cre:ERT2-SV40pA transgene was designed with the thymidine kinase (TK) minimal promoter preceded by an upstream activating sequence (UAS) 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 1382 was established on a C57BL/6J genetic background.