Cancer antigen 15-3 (CA 15-3) is a that is produced by normal breast cells. In many patients with cancerous breast tumors, there is increased production of CA 15-3 and the related cancer antigen 27.29. CA 15-3 does not cause cancer; rather, it is shed by the tumor cells and enters the bloodstream, making it useful as a tumor marker to follow the course of the cancer.
CA 15-3 is elevated in about 10% of women with early localized breast cancer and in about 70% of those with breast cancer. CA 15-3 may also be elevated in healthy people and in individuals with other cancers, conditions, or diseases, such as colorectal cancer, lung cancer, , hepatitis, and breast disease.