Schematic presentation of the structure of prototypical RING, PHD and LIM domains. The metal-ligand residues, either cysteine (C) or histidine (H), are shown as numbered spheres. Two pairs of metal ligands coordinate one zinc ion (hexagon). The numbers next to the loops connecting the metal-ligand residues indicate the minimum and maximum number of loop residues. (a) The structure of a RING domain (RING-HC type). The metal-ligand pairs 1 and 3 coordinate one zinc ion, while pairs 2 and 4 coordinate the second one in a so-called cross-brace arrangement. (b) The structure of a PHD domain reveals a cross-brace arrangement similar to the RING domain. (c) The LIM domain structure is distinct in its consecutive zinc ligation scheme: the first zinc ion is coordinated by the metal-ligand pairs 1 and 2, while the second ion is coordinated by pairs 3 and 4.