SDHB (Succinate dehydrogenase [ubiquinone] iron-sulfur subunit, mitochondrial) is an iron-sulfur protein subunit of complex II, having an intermediate role in the electron transfer from the citric acid cycle to the respiratory chain. 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. In immunohistochemistry of normal tissue, SDHB has cytoplasmic positivity in all cell types in tissues throughout the body. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804; J. Clin. Endocrinol. Metab. 91 (11): 4505–9, PMID: 16912137;