Dictionary.net

A homologous group of endogenous cysteine proteinase inhibitors. Four distinct families are recognized within the cystatin superfamily: cystatin B or stefins; cystatin C or post-gamma-globulin; egg-white or chicken cystatin; and kininogen cystatin. The cystatins inhibit most Cysteine Endopeptidases of the papain type, and other peptidases which have a sulfhydryl group at the active site.