Methylmalonic acidemia (MMA, OMIM #251000) is a prototypic organic aciduria caused by impaired activity of the 5′deoxy-adenosylcobalamin–dependent (vitamin B12–dependent) mitochondrial enzyme methylmalonyl-CoA mutase (MMUT) (MMUT or MCM, EC 5.4.99.2) (16). Here, MMUT is linked to methylmalonic acidemia.