Cytochrome b is a component of the ubiquinol-cytochrome c reductase complex, which is a respiratory chain that generates an electrochemical potential, coupled to ATP synthesis. The principal components of the b-c1 complex are cytochrome b, cytochrome c1, and the rieske protein. Cytochrome b posesses two heme groups, which are not covalently attached to the protein. Mutations in cytochrome b are associated with Leber’s hereditary optic neuropathy and with myopathy.