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 is assembled by combining five genomic exons, four very short 
amino terminal sequences and one extraordinarily large 
exon. This domain is dominated by its capacity for O-
glycosylation and its 
resultant richness in 
serine and 
threonine residues. Additionally, an amino terminal extension is present, which comprises four genomic exons. The 
amino acid composition of the amino terminal extension was found to be consistent with the 
amino acid composition of the amino terminal domain. The molecular structure is dominated by a repeat domain comprising 156 
amino acid repeat units, which encompass the 
epitope binding sites. More than 60 repeat units have been identified, sequenced, and contiguously placed in the CA125 domain structure. The repeat units encompass an interactive disulfide bridged C-
enclosure and the site of OC125 and M11 binding. The repeat sequences demonstrated 70-85% homology to each other. Expression of the repeats was demonstrated in E. coli. The CA125 molecule is anchored at its carboxy terminal through a 
transmembrane domain and a short cytoplasmic 
tail. The carboxy terminal also contains a proteolytic cleavage site approximately 50 amino acids upstream from the 
transmembrane domain, which allows for proteolytic cleavage and release of the CA125 molecule. Any one of the repeat domains has the potential for use as a new 
gold standard for detecting and monitoring the presence of the CA125 
antigen. Further, the repeat domains or other domains, especially the c-terminal to the repeat domain also provide a basis for the development of a vaccine, which would be useful for the treatment of 
ovarian cancer and other carcinomas where CA125 is elevated.