These biomarkers predict treatment response, whereas “prognostic” biomarkers inform only about prognosis irrespective of treatment.4 A predictive biomarker could be a conventional blood test (e.g., carbohydrate antigen 19-9 [CA19-9]), a genomic alteration measured in tissue, or circulating tumor DNA (ctDNA), or a gene expression signature.5 An illustrative example is the identification of KRAS mutation as a predictive biomarker in patients with metastatic colorectal cancer, signifying resistance to cetuximab.6,7 Biomarkers also can monitor treatment response. This evidence concerns the gene KRAS and metastatic colorectal cancer.