CA-125 is a protein that in humans is encoded by CA-125 gene. It is a member of the mucin family glycoproteins. CA-125 has found application as a tumor marker or biomarker that may be elevated in the blood of some patients with specific types of cancers, or other conditions that are benign. It is a membrane associated mucin that possesses a single transmembrane domain. A unique property of MUC16 is its large size. CA-125 is more than twice as long as MUC1 and MUC4 and contains about 22,000 amino acids, making it the largest membrane-associated mucin.