multicellular organisms, removal of this enzyme from the genome has also been shown to be lethal at the embryonic stage in mice. SdhA mutations can lead to Leigh syndrome, mitochondrial encephalopathy, and optic atrophy. SdhB mutations can lead to tumorogenesis in chromaffin cells, causing a class of tumors known as succinate dehydrogenase deficient including hereditary paraganglioma and hereditary pheochromocytoma, succinate dehydrogenase deficient renal carcinoma and succinate dehydrogenase deficient gastrointestinal stromal tumor (GIST). Tumors tend to be malignant. It can also lead to decreased life-span and increased production of superoxide ions. SdhC mutations can lead to decreased life-span, increased production of superoxide ions, hereditary paraganglioma and hereditary pheochromocytoma. Tumors tend to be benign. These mutations are uncommon. SdhD mutations can lead to hereditary paraganglioma and hereditary pheochromocytoma. Tumors tend to be benign, and occur often in the head and neck regions. These mutations can also decrease life-span and increase production of superoxide ions. Mammalian succinate dehydrogenase functions not only in mitochondrial energy generation, but also has a role in oxygen sensing and tumor suppression; and, therefore, is the object of ongoing research. Reduced levels of the mitochondrial enzyme succinate dehydrogenase (SDH), the main element of complex II, are observed post mortem in the brains of patients with Huntington's Disease, and energy metabolism defects have been identified in both presymptomatic and symptomatic HD patients. See also SDHA SDHB SDHC SDHD Fumarate reductase References EC 1.3