KLK3 and breast cancer: Circulating protein markers are the most established diagnostic tools for cancer, and include prostate specific antigen (PSA) for screening prostate cancer 11, cancer antigen (CA) 15-3 for postoperative follow-up of breast cancer, carbohydrate antigen 19-9 (CA 19-9), carcinoembryonic antigen (CEA) for intrahepatic cholangiocarcinoma (ICC), alpha-fetoprotein (AFP), and carbohydrate antigens for screening multiple malignancies 13, 14.