Enables cysteine-type endopeptidase inhibitor activity. Predicted to be located in cytoplasm; extracellular space; and vesicle. Human ortholog(s) of this gene implicated in several diseases, including Alzheimer's disease; CST3-related cerebral amyloid angiopathy; artery disease (multiple); and autoimmune disease of the nervous system (multiple). Is an ortholog of human CST1 (cystatin SN); CST3 (cystatin C); and CST4 (cystatin S).