Wikipedia
Prostate-specific antigen (PSA), also known as gamma-seminoprotein or kallikrein-3 (KLK3), is a glycoprotein enzyme encoded in humans by the KLK3 gene. PSA is a member of the kallikrein-related peptidase family and is secreted by the epithelial cells of the prostate gland. PSA is produced for the ejaculate, where it liquefies semen in the seminal coagulum and allows sperm to swim freely. It is also believed to be instrumental in dissolving cervical mucus, allowing the entry of sperm into the uterus.
PSA is present in small quantities in the serum of men with healthy prostates, but is often elevated in the presence of prostate cancer or other prostate disorders. The United States Preventive Services Task Force (USPSTF, 2012) does not recommend PSA screening for prostate cancer, noting that the test may result in “overdiagnosis” and “overtreatment” because "most prostate cancer is asymptomatic for life" and treatments involve risks of complications including impotence (erectile dysfunction) and incontinence. The USPSTF concludes "the potential benefit does not outweigh the expected harms." PSA is not a unique indicator of prostate cancer, but may also detect prostatitis or benign prostatic hyperplasia. 30 percent of patients with high PSA have prostate cancer diagnosed after biopsy.