MCEE and methylmalonic acidemia: Isolated methylmalonic acidemia (iMMA) refers to a complex and heterogeneous group of monogenic disorders affecting the metabolism of methylmalonate and cobalamin (Cbl, vitamin B12), characterized by elevated concentrations of methylmalonic acid (MMA) in blood and urine [1,2], which arises from a partial or complete deficiency of the enzyme methylmalonyl-CoA mutase (MUT), a defect in the transport or synthesis of its cofactor 5-deoxyadenosyl-cobalamin (AdoCbl) (cblA, cblB, cblD-MMA), or a defect in the enzyme methylmalonyl-CoA epimerase (MCEE) [3,4].