Cancer antigen 125 (CA125) is a protein in humans encoded by the MUC16 gene and found on the cell membrane of ovarian cancer cells. CA125 is the only tumor marker recommended for clinical use in the diagnosis and management of ovarian cancer. CA125 levels may also be high in non-cancerous conditions, including pelvic inflammatory disease, any condition causing excessive fluid in the abdomen such as liver disease, and in healthy women during pregnancy and menstruation. Diagnostic assays for CA125 are based on the antibody sandwich principle using ELISA or CLIA.