Carcinoembryonic antigen (CEA) is a nonspecific serum biomarker, since it appears at elevated levels in different malignant cancers, including colorectal cancer, medullary thyroid cancer, breast cancer, mucinous ovarian cancer, pancreatic cancer, lung cancer, and advanced stages of GC. The gene discussed is CEACAM5; the disease is medullary thyroid gland carcinoma.