Carcinoma antigen 125 or glycan antigen 125 (CA125), also known as mucin 16 or MUC16, is a protein that in humans is encoded by the MUC16 gene.MUC16 is a member of the mucin family of glycoproteins. It is a component of the epithelium of the ocular surface, respiratory tract, and female reproductive tract. Because MUC16 is highly glycosylated, it forms a hydrophilic environment in the apical membrane of epithelial cells, which serves as a lubricating barrier against foreign particles and infectious agents. In addition, the cytoplasmic terminus of MUC16 has been shown to interact with the cytoskeleton by binding to members of the ERM family of proteins.MUC16 expression is altered in dry eye, cystic fibrosis, and several cancers.