Isolated methylmalonic acidemia (MMA) is caused by an extreme deficiency or a lack of activity of the methylmalonyl-CoA mutase (MCM) enzyme, which needs adenosylcobalamin (AdoCbl) as a cofactor, whereas combined methylmalonic acidemia and homocystinuria is caused by elevated plasma homocysteine and decreased levels of the coenzymes AdoCbl and methylcobalamin (MeCbl; Fig. 1) [2]. The gene discussed is MMUT; the disease is methylmalonic acidemia.