An enhancer trap transgenic approach was used to generate these mice. The TK-bgi-cre:ERT2-bGHpA transgene was designed with the thymidine kinase (TK) minimal 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). A beta globin intron separates the promoter from the cre gene and the bovine growth hormone polyadenylation signal follows the cre sequence. This transgene was injected into C57BL/6J embryos. Founder line 21 was established on a C57BL/6J genetic background.