The most popular biomarkers used in clinical practice are the prostate-specific antigen (PSA) for prostate cancer; the carcinoembryonic antigen (CEA) for gastrointestinal, breast, and lung cancer; CA125 for ovarian cancer; CA19-9 for pancreatic cancer; and CA15-3 for breast cancer [15]. The gene discussed is CEACAM5; the disease is lung cancer.