- The ecto-5'-nucleotidase subunits in dimers are not linked by disulfide bridges but by non-covalent bonds. Martínez-Martínez, A., Muñoz-Delgado, E., Campoy, F.J., Flores-Flores, C., Rodríguez-López, J.N., Fini, C., Vidal, C.J. Biochim. Biophys. Acta (2000)