The endogenous locus was replaced with a cassette that contained a 5' loxP site, a copy of the endogenous locus, an frt-flanked neo cassette with a 3' loxP site, an open reading frame with an amino acid substitution that resulted in an amino acid substitution of arginine for glycine at position 138 (G138R) and an IRES-eGFP. Only cre-mediated recombinaton can remove the wild-type locus and result in expression of the locus with the nucleotide substitution. (J:132032)