Serum prostate-specific antigen (PSA) needs to be monitored with ultrasensitive PSA assays (uPSAs) for oncologists to be able to start salvage radiotherapy (SRT) while PSA is <0.5 μg/L for patients with prostate cancer (PCa) relapsing after a radical prostatectomy (RP). This evidence concerns the gene KLK3 and prostate cancer.