Methylmalonic acidemia (MMA, MIM#251000) and propionic acidemia (PA, MIM#606054) are autosomal recessive organic acidemias that are characterized by the accumulation of methylmalonate or propionate due to a defect in either methylmalonyl-CoA mutase (MUT) or propionyl-CoA carboxylase (PCC). The gene discussed is MMUT; the disease is methylmalonic acidemia.