[5][6] The cystatin superfamily encompasses proteins that contain multiple cystatin-like sequences.

This gene encodes a stefin that functions as an intracellular cysteine protease inhibitor.

The protein is able to form a dimer stabilized by noncovalent forces, inhibiting papain and cathepsins L, H and B.

The protein is thought to play a role in protecting against the proteases leaking from lysosomes.

Evidence indicates that mutations in this gene are responsible for the primary defects in patients with Unverricht–Lundborg disease, a form of progressive myoclonic epilepsy (EPM1).