CA125, a glycoprotein detectable from epithelial ovarian cancer antigens by Bast et al. in 1983 that can be bound by the monoclonal antibody OC125, is derived from the embryonic developing somatic epithelium and is not present in normal ovarian tissue, therefore it is most commonly found in the serum of patients with epithelial ovarian tumors (plasmacytoma), and its diagnostic sensitivity is high but its specificity is poor. It is not present in mucinous ovarian tumors. Serum CA125 is elevated in 80% of patients with epithelial ovarian tumors, but not in nearly half of early cases, so it is not used alone for the early diagnosis of epithelial ovarian cancer. 90% of patients have serum CA125 independent of disease progression, so it is mostly used for disease detection and efficacy assessment. 95% of healthy adult women have CA125 levels ≤ 40 U/ml, and an elevation of more than twice the normal value should cause If the CA125 level is more than twice the normal value, it should be taken seriously. CA125 is not only a specific marker for ovarian cancer, but also for adenocarcinoma of the fallopian tube, endometrium, cervix, pancreas, intestine, breast and lung. CA125 is an antigen recognized by a monoclonal antibody (named OC125) obtained by immunizing mice with ovarian plasmacytoid cystic adenocarcinoma cells and hybridizing them with myeloma cells, and is a transmembrane glycoprotein of 5797 base pairs located on chromosome 19p13.2, which belongs to IgG1. CA125 has a relative molecular mass of 200,000 to 1,000,000 and is a macromolecular glycoprotein with a cyclic structure containing 24% glycans, a mucin-like glycoprotein complex that belongs to IgG. The concentration of CA125 in healthy adults is less than 35 U/mL. The concentration of CA125 in healthy adults is less than 35 U/mL. Clinical significance: (1) The serum CA125 level is significantly elevated in patients with ovarian cancer, and it decreases rapidly in those with effective chemotherapy and surgery. In case of recurrence, CA125 elevation may precede clinical symptoms. (2) Other non-ovarian malignancies also have a certain positive rate, such as breast cancer 40%, pancreatic cancer 50%, gastric cancer 47%, lung cancer 44%, colorectal cancer 32%, and other gynecological tumors 43%. For lung cancer patients, the positive rate of CA125 increases significantly with the progression of lung cancer stage. Clinical studies show that the positive rate of lung cancer stage I is 7.8%, stage II is 18.6%, stage III is 32.5%, stage IV is 53.9%, and the overall positive rate is 26.9% (3) Non-malignant tumors, such as endometriosis, pelvic inflammatory disease, ovarian cysts, pancreatitis, hepatitis, and (3) Non-malignant tumors, such as endometriosis, pelvic inflammatory disease, ovarian cysts, pancreatitis, hepatitis, liver cirrhosis, etc. were found to be elevated to varying degrees, but the positive rate was low. (4) Elevated CA125 is found in the thoracoabdominal fluid and higher concentrations of CA125 can be detected in amniotic fluid. (5) Elevated CA125 may also be found in the first trimester of early pregnancy. It is present in epithelial ovarian cancer tissues and patient’s serum, and is mainly used as an aid to diagnose malignant plasma ovarian cancer, epithelial ovarian cancer, and also as an indicator for observing the efficacy of ovarian cancer after surgery and chemotherapy. It can be continuously and dynamically observed.