KLK3 and neoplasm: To overcome this shortcoming, other CTC-specific marker genes were identified, such as tumour-specific antigen 9 (TSA-9); cytokeratin (CK) 18, 19, 20; pre-progastrin-releasing peptide (Pre-proGRP) [83,84]; as well as tumour immune markers including EGFR, HER2, and prostate-specific antigen (PSA) [85].