Inhibits various C1 cysteine proteases including cathepsin L, papain and cathepsin B. This protein has no toxic activity and its function in the venom is unknown. It may play a role as a housekeeping or regulatory protein (By similarity). Expressed by the venom gland at an extremely low level (at protein level). The recombinant protein does not inhibit calpain-1 (CAPN1), a C2 family cysteine protease and legumain (LGMN), a C13 family cysteine protease. Does not provoke cell death (PC3 prostate cancer cells) (By similarity). Belongs to the cystatin family.