MUC16, also known as the CA125 antigen, is a mucin protein that may be found in type I transmembrane or secreted forms that are used monitor the progress of epithelial ovarian cancer therapy. MUC16 is over-expressed by tumor cells including ovarian and mesothelial cancers. The transmembrane form can adhere to mesothelin in the peritoneum, facilitating metastasis of ovarian cancer to the peritoneal cavity. MUC16 also binds galectin-1 on immune cells and enhances its expression on tumor cells. MUC16-expressing tumors adhere to NK cells, down-regulate CD16 and suppress NK response, which may promote immune evasion. MUC16 is also cyclically expressed in the endometrium and may contribute to immune privilege during pregnancy. In the eye, MUC16 and other mucins protect the cornea and keep it hydrated. It is altered on the conjunctival epithelium of patients with non-Sjogren dry eye syndrome.