This is a glycoprotein found in the cytoplasm of epithelial cells of prostate (a gland in the base of the penis). Its level in the blood may be elevated in men who have a benign or a malignant growth in the prostate. For this reason, the PSA test does not always distinguish between benign prostate conditions (which are very common in older men) and cancer. However, its level is useful as a screening test for early detection and to monitor the effectiveness of prostate cancer treatment over time. Normal range: less than 4 ng/mL