KLK3 and breast cancer: Of relevance, single screening biomarkers such as prostate-specific antigen (PSA) for prostate cancer, the cancer antigen 125 (CA125) for ovarian cancer, the cancer antigen 15-3 (CA15-3), the carbohydrate antigen 19-9 (CA19-9), and the carcinoembryonic antigen (CEA) for breast cancer are routinely used in clinical cancer diagnostics [11,12,13,14].