Using a BAC recombineering method, a targeting construct containing an insertion of an Frt-flanked neomycin selection cassette followed by an internal ribosomal entry (tires) sequence fused to a Cre recombinase transgene (iCre) between the STOP codon and the polyadenylation of the Irs4 gene. This construct was used to isolate a targeting plasmid with the neo-iCre cassette with upstream and downstream Crh homology arms. ES cells were targeted and used for blastocyst injection. Chimeras were bred for germline transmission. The selection marker was excised by breeding to Flp deleter animals.