Human Cystatin A （CSTA or Stefin A） belongs to family 1 of the cystatin superfamily， which is characterized by the lack of disulphide bonds and carbohydrates. CSTA is an intracellular inhibitor regulating the activities of cysteine proteases of the papain family such as Cathepsins B， H and L. Cystatin A has also been implicated in several disease states. Because of altered proteolytic state in cancer progression， CSTA may have a role in the proteolitic pathways.
