<p>A disulphide knot is a domain defined by a number of critical cysteine residues. The domain is restricted to the arthropoda and found in varying copy numbers (from one to five in Drosophila proteins). It is always found N-terminal to the chymotrypsin serine protease domain, which belong to MEROPS peptidase family S1A. Disulphide knots have previously been suggested to be important for dimerisation, and they appear to consist of an internal ring of 8 amino acids joined by di-sulphide bonds, at least one of which passes through the centre of the ring to form the "knot" [<cite idref="PUB00007971"/>]. </p> <p>The CLIP domain is present in horseshoe crab proclotting enzyme N-terminal domain and silkworm prophenoloxidase-activating enzyme [<cite idref="PUB00009689"/>]. It may beresponsible for mediating specific protein-protein interactions, and as such is useful for regulating cascades of serine protease activities.</p>