Cystatins S, SN and SA were first isolated from human saliva and later identified as a cysteine protease inhibitors and their role in controlling cysteine proteases derived from bacteria has been suggested. [99] [100] [101] [102] These three proteins with similar sequence (88% identity at the protein level) are distantly related to cystatin D (less then 60% identity at the protein level with cystatins S, SN and SA).