Sequences encoding a cre/ERT2 fusion protein, Frt-flanked neo cassette, and a polyadenylation signal were inserted into a BAC containing the Ucp1 promoter by recombineering methods. The neo marker was not removed, as the original construct showed more efficient cre activity. The purified construct was microinjected into fertilized C57BL/6N oocytes. Line 426 was established.