CA-125 (cancer antigen 125, carcinoma antigen 125, or carbohydrate antigen 125) also known as mucin 16 or MUC16 is a protein that in humans is encoded by the MUC16 gene. MUC16 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,[4] or other conditions that are benign.

CA-125 is the most frequently used biomarker for ovarian cancer detection. Reasons for this include evidence that ambiguous test results are more likely to lead to further invasive, harmful, and unnecessary health care than they are likely to detect ovarian cancer in women who are at average risk of developing it.