Prostate-specific antigen (PSA), also known as gamma-seminoprotein or kallikrein-3 (KLK3), is a 33 kDa glycoprotein that is mainly produced by the epithelial cells of the prostate. PSA has been also found to be expressed in many non-prostatic normal or tumor tissues, like breast or breast cancer tissue and breast cancer cells, colon, kidney, liver, parotid, lung, endometrium, periuretral, thyroid and apocrine sweat glands. PSA is present in serum in different molecular isoforms, free or unbound, and bound to serine protease inhibitors. Major part of serum PSA is bound to α-1-antichymotrypsin or α-2-macroglobulin. The normal prostate contains PSA levels about million-fold higher than serum. The normal serum levels of PSA in males are under 4 µg/l, while the PSA levels in the seminal fluid are 10 6 – fold higher, 0.2–5 g/l. The PSA concentration in female serum is much lower than in male, 0.2–0.3 µg/l. PSA is currently used for prostate cancer diagnosis and monitoring of patients with prostate adenocarcinoma. Also about 30% of female breast tumors produce a 33 kDa glycoprotein that has striking similarities to seminal PSA. PSA coud be used as a biomarker for breast cancer prognosis, for the spreading of hematogenous micrometastases.