While existing follow-on tests contingent on elevated PSA, so-called PSA reflex tests [16,17], and multiparametric MRI [18] reduce at least a fraction of unnecessary biopsies in men with PSA levels of 4–10 ng/mL, there is still a significant unmet clinical need for improved reflex tests capable of more accurately discriminating prostate cancer’s presence from other causes of elevated PSA while minimizing false negatives. The gene discussed is KLK3; the disease is prostate cancer.