Among the reasons are the following:Some markers are tissue-specific (such as prostate-specific antigen (PSA)) and their elevation in serum is due to leakage of the highly abundant PSA molecules in prostate cells (benign or malignant) into the adjacent blood vessels.15 Circulating tumor DNA is a similar example of an elevation of a biomarker in nearly all patients with cancer, especially at late stages.16 This test is highly sensitive and specific, since the DNA leaks from dying tumor cells. Here, KLK3 is linked to cancer.