Estimates have suggested that 96–100% of patients with stage IV PrCa have increased serum PSA levels, while a prospective study on serum PSA levels showed that a primary lesion was found in 50–80% of CUP patients with serum PSA > 10 ng/mL, and distant metastases were observed among those with serum PSA > 100 ng/mL [15–17]. Here, KLK3 is linked to pure red-cell aplasia.