Cancer antigen 15-3 (CA15-3) or transmembrane glycoprotein mucin 1 (MUC1) is an epitome of a large transmembrane glycoprotein named MUC1 derived from the MUC1 gene. MUC1 protein is frequently over-expressed and aberrantly glycosylated on its extracellular region in cancer, and it is the most widely used serum marker in breast cancer. The main uses of CA15-3 are in preclinically detecting recurrent breast cancer and monitoring the treatment of patients with advanced breast cancer. However, physiologically, MUC1 appears to play a role in cell adhesion, and the high levels present in cancer may be linked to metastasis. CA15-3 assays are based on the two-site sandwich ELISA or CLIA assay principle.