mitochondrial complex II deficiency, nuclear type 1 (MONDO_0100294, a Monarch Disease Ontology identifier) can be described as follows. Complex II deficiency is a mitochondrial disease. Mitochondria are specialized compartments in cells that create more than 90% of the energy needed by the body. In mitochondrial diseases, the mitochondria don't work correctly resulting in less energy in the cell, cell injury and cell death. The signs and symptoms of mitochondrial complex II deficiency can vary greatly from severe life-threatening symptoms in infancy to muscle disease beginning in adulthood. Complex II deficiency can be caused by mutations in the SDHA, SDHB, SDHD, or SDHAF1 genes. In many cases the underlying gene mutations cannot be identified. Complex II deficiency is inherited in an autosomal recessive fashion. Complex II deficiency gene mutation carriers may be at an increased risk for certain cancers. Also known as: isolated mitochondrial respiratory chain complex II deficiency, isolated succinate-CoQ reductase deficiency, isolated succinate-coenzyme Q reductase deficiency, isolated succinate-ubiquinone reductase deficiency, mitochondrial complex II deficiency.