MUC16 is a member of the mucin family glycoproteins. Mucins are high molecular weight, O-glycosylated proteins that play an important role in forming a protective mucous barrier, and are found on the apical surfaces of the epithelia. MUC16 is a membrane-tethered mucin that contains an extracellular domain at its amino terminus, a large tandem repeat domain, and a transmembrane domain with a short cytoplasmic domain. The amino terminus is highly glycosylated, while the repeat region contains 156 amino acid repeats unit that are rich in serines, threonines, and prolines. Interspersed within the repeats are Sea urchin sperm protein Enterokinase and Agrin (SEA) modules, leucine-rich repeats and ankyrin (ANK) repeats. These regions together form the ectodomain, and there is a potential cleavage site found near an SEA module close to the transmembrane domain. This protein is thought to play a role in forming a barrier, protecting epithelial cells from pathogens. MUC16 has been used as a marker for different cancers, with higher expression levels associated with poorer outcomes.

MUC16 is a membrane bound glycoprotein involved in the progression and metastasis of pancreatic and ovarian cancer. The protein is shed into the serum and the resulting cancer antigen 125 (CA125) can be detected by immunoassays. The CA125 epitope is used for monitoring ovarian cancer treatment progression, and has emerged as a potential target for antibody mediated immunotherapy.