The cystatin superfamily is a well-established family of cysteine protease inhibitors. All true cystatins inhibit cysteine peptidases of the papain family, such as cathepsins, while some also inhibit legumain family enzymes. CST9L (Cystatin-9-like) is a 147 amino acid secreted protein that is related to cystatin 9, which is thought to play a role in hematopoietic differentiation or inflammation.