Mutations in genes encoding for either structural subunits or assembly factors have been described (SDHA, SDHB, SDHD, and SDH assembly factor 1 (SDHAF1) for mitochondrial diseases; SDHD, SDHC, SDHB, SDHA, and SDHAF2 for hereditary paragangliomas). The gene discussed is SDHB; the disease is mitochondrial disease.