Human saliva appears to contain several cysteine proteinase inhibitors that are immunologically related to cystatin S but that differ in their specificity due to amino acid sequence differences. Cystatin SN, with a pI of 7.5, is a much better inhibitor of papain and dipeptidyl peptidase I than is cystatin S, although both inhibit ficin equally well. Expressed in submandibular and sublingual saliva but not in parotid saliva (at protein level). Expressed in saliva, tears, urine and seminal fluid. Variant Leu-31. Belongs to the cystatin family.