Enables cysteine-type endopeptidase inhibitor activity and protease binding activity. Involved in several processes, including negative regulation of peptidase activity; response to chromate; and response to protozoan. Located in extracellular space and secretory granule. Orthologous to several human genes including CST5 (cystatin D); CST4 (cystatin S); and CST2 (cystatin SA). [provided by Alliance of Genome Resources, Apr 2022]