Thiol protease inhibitor. Has papain inhibitory activity in vitro. May be involved in immune responses against invading Gram-negative bacteria. Widely expressed. Highly expressed in liver and to a lesser extent in spleen, gill, brain, intestine, kidney, head kidney and blood. Lowest level in muscle. By bacterial infection. E.tarda bacteria causes significant up-regulation in head kidney between 12 hours and 24 hours post-infection and in spleen between 24 hours and 48 hours post-infection. Belongs to the cystatin family.