KLK3 and cancer: When considering conventional tumour biomarkers employed in liquid biopsies to regularly screen cancers in clinic, such as the CA15-3 (breast cancer), CA19-9 (pancreatic cancer and CRC), CA125 (ovarian cancer), CEA (CRC) and prostate-specific antigen (prostate cancer), certain conditions are required, including abundant protein expression levels, high-affinitive detection reagents, and high or late stages of cancer samples, whereas low specificity, weak sensitivity, and poor accurate rates were often displayed [10].