A loxP site flanked hygromycin resistance gene and STOP cassette was inserted 100 bp upstream of a Kras mutagenized exon 2 in which valine codon 14 (GTA) was changed to isoleucine (ATA) (p.V14I). Cre-mediated recombination in the germline removed the floxed LSL cassette, allowing expression of the mutant allele.