PSA is an enzyme that helps liquefy semen after a man ejaculates. Liquifying the semen frees sperm cells so they can swim to an egg and fertilize it. PSA’s function is so essential for mammalian reproduction that prostate cancer has evolved a multitude of ways to avoid the body’s own defences against cancerous prostate cells. As a testimony to how important PSA is to reproduction, highly mutated prostate cells still produce PSA. This makes the PSA test a good clinical tool for tracking the progression of the disease. At the same time, it also is an ominous sign of how hard it will be to find a simple cure for prostate cancer.