CA15-3 is a tumor marker, also known as glycoantigen 15-3, which is the most important specific marker for breast cancer and is produced by tumor tissue. The concentration of glycoprotein CA15-3 in human serum or plasma is quantified by chemiluminescent enzyme immunoassay, and the increase is mostly seen in breast cancer, which can be used for the observation of breast cancer efficacy and prognosis estimation. It is also elevated to varying degrees when metastatic ovarian cancer, liver cancer, colon cancer, bile duct cancer, pancreatic cancer, and lung cancer are present.