The three-dimensional structure of human chorionic gonadotropin shows that each of its two different subunits has a similar topology, with three disulphide bonds forming a cystine knot. This same folding motif is found in some protein growth factors. The heterodimer is stabilized by a segment of the β-subunit which wraps around the α-subunit and is covalently linked like a seat belt by the disulphide Cys 26–Cys 110. This extraordinary feature appears to be essential not only for the association of these heterodimers but also for receptor binding by the glyco-protein hormones.