A cassette containing a nuclear localization signal, a mutated cre gene (nsCre) in which an in-frame nonsense mutation (TGA) was introduced into the position 837 of the coding region of the wild-type Cre recombinase gene, followed by frt-flanked neo gene was inserted just 5' of the initiation codon of the Agrp gene. Mutant mRNAs transcribed from nsCre transgene can be efficiently translated into full-length, functional Cre recombinase in the presence of nonsense suppressors such as aminoglycosides or synthetic homologs. (J:232044)