Background：

Complex II of the respiratory chain， which is specifically involved in the oxidation of succinate， carries electrons from FADH to CoQ. The complex is composed of four nuclear-encoded subunits and is localized in the mitochondrial inner membrane. The iron-sulfur subunit is highly conserved and contains three cysteine-rich clusters which may comprise the iron-sulfur centers of the enzyme. Sporadic and familial mutations in this gene result in paragangliomas and pheochromocytoma， and support a link between mitochondrial dysfunction and tumorigenesis.