Methylmalonic acidurias (MMA) include a heterogeneous group of autosomal recessive genetic disorders caused by deficiency of the mitochondrial enzyme methylmalonyl-CoA mutase (MCM) or by defects in cobalamin (cbl) absorption, intracellular cbl trafficking, or synthesis of adenosylcobalamin (Adocbl) and methylcobalamin (Mecbl), which serve as cofactors for MCM and for the cytosolic enzyme methionine synthase, respectively (Figure 1). This evidence concerns the gene MMUT and Methylmalonic aciduria.