Serial measurement of serum tumor markers—including carcinoembryonic antigen (CEA) for gastrointestinal cancers, carbohydrate antigen 19–9 (CA 19–9) for pancreatic cancer, cancer antigen 125 (CA 125) for ovarian cancer, and cancer antigen 27–29 (CA 27–29) for breast cancer—can demonstrate response to cancer treatment or signal recurrent disease. This evidence concerns the gene CEACAM5 and breast cancer.