ca 15-3 ca 15-3 ca 15-3, for carcinoma antigen 15-3, is a tumor marker for many types of cancer, most notably breast cancer. it is derived from muc1. ca 15-3 and associated ca 27-29 are different epitopes on the same protein antigen product of the breast cancer-associated muc1 gene. elevated ca15-3, in conjunction with alkaline phosphatase (alp), was found to be associated with an increased chance of early recurrence in breast cancer. both ca 15-3 and ca 27-29 may be elevated in patients with benign ovarian cysts, benign breast disease, and benign liver disease. elevations may also be seen in cirrhosis, sarcoidosis and lupus.