The 206 kb C57BL/6J mouse bacterial artificial chromosome (BAC) RP23-370M9, containing the entire Cyp39a1 locus (and other genes), was modified by targeting a Cre recombinase coding sequence (the CreERT2 sequence modified back to encode the normal Cre recombinase gene), SV40 polyA signal, and frt-flanked neomycin cassette to the ATG start site of the Cyp39a1 locus. The targeted BAC sequences were further modified using FLP recombination to remove the selection cassette and linearized to remove its original vector backbone.