CA15-3, for Carcinoma Antigen 15-3, is a tumor marker for many types of cancer, most notably breast cancer.1 It is derived from MUC1.2 CA15-3 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.3
