Methylmalonic acidemia (MMA; MIM #251000)—the most common form of organic acidemias—is caused by inactivating mutations in the MMUT gene encoding methylmalonyl-CoA mutase (MMUT), which is a (vitamin B12-dependent) mitochondrial enzyme that catabolizes branched amino acids and certain lipids to succinyl-CoA, ultimately feeding the tricarboxylic acid (TCA) cycle and energy production. The gene discussed is MMUT; the disease is methylmalonic acidemia.