Complex III or quinol-cytochrome c reductase performs electron transfer coupled to proton pumping. It occurs as a symmetrical, tightly bound dimer (referred to as cIII2). This dimerisation occurs prior to the pre CIII complex formation, annotated as pre-CIII(2). Each monomer is composed of a catalytic core containing MT-CYB, CYC1 and UQCRFS1. An additional seven subunits are required for stability and/or correct assembly of the enzyme, but are not involved in the catalysis, but .