A BAC (RP23-181D14) containing the Bmi1 locus was modified via recombineering methods by insertion of an IRES-cre/ERT2-pA-Frt-neo-Frt cassette at 85 base pairs 3' of the stop codon of Bmi1. The neo cassette was deleted by transforming the modified BAC into arabinose-induced cells expressing Flp. The final vector was injected into C57BL/6 blastocysts to generate germline-transmitting mice. (J:177145)