Proteins are the direct executors of biological activities, and some of them have been used as clinical biomarkers for various diseases, e.g., thyroglobulin (Tg) and thyroid peroxidase (TPO) for thyroid diseases, prostate-specific antigen (PSA) for prostate cancer, carcinoembryonic antigen (CEA) for liver cancer, and HER-2 for breast cancer subtyping. The gene discussed is CEACAM5; the disease is breast cancer.