A selection cassette containing an internal ribosomal entry sequence (IRES) linked to Cre-recombinase and an Frt-flanked neomycin resistance gene was targeted 3 bp after the stop codon of the Sim1 gene, so that Cre-recombinase expression was driven by the endogenous gene. Flpe recombinase transgene mice were then used to remove the neomycin selection cassette.