KLK3 and breast cancer: A number of FDA (US Food and Drug Administration)-approved plasma protein biomarkers for cancer are currently used in clinical practice, such as prostate-specific antigen (PSA) for prostate cancer, alpha-fetoprotein (AFP) for testicular cancer, cancer antigen 125 (CA-125) for ovarian cancer, CA 15-3 for breast cancer and CA 19-9 for pancreatic cancer (3, 4).