Prostate-specific antigen (PSA) is a protein produced by the prostate gland in small amounts. It is one of the most common biomarkers for prostate cancer and is used for diagnosis, monitoring, and follow-up of men with prostate cancer.  However, PSA levels alone are not sufficient to make a definitive diagnosis of prostate cancer, and additional tests such as digital rectal exam (DRE) and prostate biopsy are needed.