Cystine knots.

Recent reports of the crystal structure of the glycoprotein hormone human chorionic gonadotrophin show that cystine knots are proving to be versatile structural motifs that enable the construction of a variety of proteins with different functional properties. Because of their shape, there appears to be an intrinsic requirement for the cystine-knot growth factors to form dimers. This extra level of organization increases the variety of structures built around this simple structural motif.