Prostate-specific antigen (PSA) is a valuable prostatic cancer biomarker that is now widely used. for population screening, diagnosis, and monitoring of patients with prostate cancer. One of the major advantages of PSA as a tumour marker is its tissue specificity. However, more recently, it was confirmed that PSA is widely expressed, at lower concentrations than in prostate in many tissues. There is increasing evidence that PSA can be detected immunohistochemically in a variety of steroid hormone-stimulated normal, and malignant epithelial tissues (Fan et al., 2000). Especially in the female breast, PSA has been found in all non-pathological and pathological breast secretions, tissue extracts and fluids. These new findings do not limit the value of PSA in prostate cancer diagnostics but may expand its applications to breast cancer (Diamandis, 2000).