Cysteine protease inhibitor which inhibits members of the peptidase C1 family. Also acts as an asparaginyl endopeptidase inhibitor (PubMed:11301256, PubMed:15664654). In the human host, inhibits CTSL/cathepsin L, CTSS/cathepsin S, CTSB/cathepsin B and asparaginyl endopeptidase LGMN/AEP which may cause defects in both antigen and MHC class II invariant chain CD74/Ii processing (PubMed:11301256). Expressed in all life cycle stages. Belongs to the cystatin family.