Recent reports of the crystal structure of the glycoprotein hormone human chorionic
gonadotrophin shown taht cystine knots are proving to be versatile structural motifs that
enable the construction of variety of proteins with different functional properties. Because of
their shape, there appears to be an intrisic 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