Inhibits various C1 cysteine proteases including cathepsin L (Ki is 0.1 nM), papain (Ki is 0.19 nM), cathepsin S (Ki is 1.2 nM), and cathepsin B (Ki is 2.5 nM). This protein has no toxic activity and its function in the venom is unknown. It may play a role as housekeeping or regulatory protein. Expressed by the venom gland. Does not inhibit calpain. Belongs to the cystatin family.