Exon 5 was replaced with a modified one in which nucleotide substitutions (GA-to-AG) result in the amino acid substitution of glutamic acid with arginine at position 177 (p.E177R), mimicking a mutation found in some human tumors. Cre-mediated recombination removed the loxP site flanked puromycin resistance gene cassette inserted into intron 1. (J:198482)