MSH2 and Neoplasm of the lung: The complex nature of the genomic instability that characterizes Msh2-null mice might suggest that in the double mutant K-rasLA1/+; Msh2−/− mice, either K-rasLA1/+ recombination events were occurring at a higher frequency than in mice with K-rasLA1/+ alone, or that additional mutations in key growth control genes were stimulating lung tumor development and possibly progression.