The success of therapy and follow-up of prostate cancer patients is traditionally evaluated by PSA levels and bone scintigraphy findings, but now it is shifting toward the measurement of blood biomarkers of DNA methylation in GSTP1 (glutathione S-transferase pi 1) and APC (adenomatous polyposis coli), both of which have a prognostic role [129]. The gene discussed is KLK3; the disease is prostate cancer.