The CA125 gene has been cloned and multiple repeat sequences as well as the carboxy terminus have been identified. The CA125 molecule comprises three major domains: an extracellular amino terminal domain (Domain 1); a large multiple repeat domain (Domain 2); and a carboxy terminal domain (Domain 3) which includes a transmembrane anchor with a short cytoplasmic domain. The amino terminal domain has numerous O-glycosylation sites. Domain 2 is a repeat domain comprising homologous repeat units of 156 amino acids. More than 60 repeat units have been identified, sequenced, and contiguously placed in the CA125 domain structure. The repeat units are the sites of OC125 and M11 antibody binding. The CA125 molecule is anchored at its carboxy terminal through a transmembrane domain and a short cytoplasmic tail.