If your loved one is under breast cancer treatment, it is normally overwhelming with the number of the test by doctors to monitor breast cancer. Here the Cancer-Antigen-15-3 is a type of protein, which is a normal product tissue of the breast. This protein never causes the breast cancer when a cancerous tumor is higher in your breast and if you l CA15-3 level may develop as the number of the cancer cell in your breast and even it has a chance to spread over part of the body. Hence, you must, you must take right treatment to avoid such the problem in the breast.