Enables peptidase inhibitor activity and protease binding activity. Involved in several processes, including animal organ development; circadian sleep/wake cycle, REM sleep; and embryo implantation. Located in several cellular components, including multivesicular body; neuronal cell body; and nuclear membrane. Biomarker of intracranial aneurysm; iron deficiency anemia; and obesity. Human ortholog(s) of this gene implicated in CST3-related cerebral amyloid angiopathy and age related macular degeneration 11. Orthologous to several human genes including CST3 (cystatin C). [provided by Alliance of Genome Resources, Apr 2022]