KLK4 and neoplasm: Belonging to the Kallikrein family, PSA, thought to be expressed only in prostatic tissue, is now known to show tissue distribution in the breast, cervix, placenta, lungs, heart, etc. Its status as a tumor marker of the prostate gland has also changed over time because infection, inflammation, injury, or malignancies in the tissues where it is distributed can lead to a rise in PSA levels [47].