For example, even though it is clearly understood that tumors with mutations in EGFR harbor many variants in genes other than EGFR, mutations in EGFR are used as a biomarker for treating lung adenocarcinomas with therapies such as Gefitinib and Erlotinib. In addition, evidence suggests that EGFR and KRAS mutations occur in a mutually exclusive manner and that tumors with KRAS mutations fail to respond to these drugs [39]. This evidence concerns the gene EGFR and lung adenocarcinoma.