An IRES and loxP site flanked neomycin resistance gene and STOP cassette were inserted along with a constitutively active Kras cDNA with the amino acid substitution of glycine with aspartic acid at position 12 (p.G12D) into intron 1. Cre-mediated removal of the STOP cassette is required to achieve expression of the constitutively active Kras. (J:217168)