BRAF and neoplasm: As a BrafV637E mutation had not previously been ascribed to urethane carcinogenesis, but has been detected in liver or lung tumors induced by other carcinogens or arising spontaneously in mice [29], we confirmed that this mutation was indeed somatic by Sanger sequencing exon 18 of Braf from the tumor and matched lung (normal) tissue (S2 Table), and further, not a product of p53 loss, as the same mutation was detected in urethane-induced tumors in a p53 wild-type background (Fig 4A).