KLK3 and prostate cancer: In view of the more and more extensive application of non-invasive liquid biopsy in early diagnosis of tumorigenesis as well as tumor metastasis, great efforts have been made to identify useful biomarkers for liquid biopsy to evaluate the possibility and the progressiveness of prostate cancer, such as [-2]pro-prostate specific antigen, prostate health index, and prostate cancer antigen 3 26-29.