The 3-methylglutaconic acidurias (MGA) are a group of congenital metabolic disorders characterized by defects in mitochondrial function and excessive accumulation of 3-methylglutaconic acid and 3-methylglutaric acid. Five different types of MGA are known, all phenotypically differing from each other, and resulting from defects in different genes. MGA type I is typified by metabolic acidosis, psychomotor and speech delay, dystonia, spastic quadriparesis, seizures, basal gangliar atrophy, hepatomegaly, and severe hypotonia. Only about 20 cases of this condition have been reported worldwide.