CA-15.3 (Cancer Antigen 15.3) is a tumor marker primarily used in breast cancer management. It helps in tracking disease progression, evaluating response to treatment, and detecting recurrence, especially in metastatic breast cancer cases. Although it is not used for early diagnosis, persistently high or rising levels may indicate tumor growth. CA-15.3 can also be elevated in other conditions such as liver disease, lung cancer, and ovarian cancer, reducing its specificity. It is most effective when combined with other diagnostic tests, imaging studies, and clinical evaluations.