Methylmalonic acidemia (MMA; MIM #251000) — the most common form of organic acidemias — is due to recessive, inactivating mutations in the MMUT gene encoding methylmalonyl-CoA mutase (MMUT) — a (vitamin B12-dependent) mitochondrial enzyme that catabolizes branched chain amino acids and certain lipids to succinyl-CoA, thereby feeding the tricarboxylic acid (TCA) cycle and energy metabolism. Here, MMUT is linked to inborn organic aciduria.