https://www.folate-analogue.com/index.php/2016/12/09/cystatin-c-is-an-active-cysteine-protease-inhibitor-and-is-the-most-abundant-cysteine-protease-inhibitor-of-the-cystatin-c-superfamily-and-is-found-in-all-body-fluids-and-is-expressed/