CA 15-3 is a mucin transmembrane glycoprotein expressed at the apical surface epithelium of the mammary gland, female reproductive tract, and stomach. It comprises three structural domains: a large and heavily glycosylated extracellular segment (exo-domain), a hydrophobic type 1 transmembrane region, and a short cytoplasmic tail domain involved in several signalling processes. An increase in the serum CA 15-3 shed ectodomain is associated with the progression of carcinoma in patients. CA 15-3 is a tumor marker primarily in the management of breast cancer. It is used as a positive control in the ELISA test kit for the detection of breast cancer.