Following this step, methylmalonic acidemia and homocystinuria type D protein (MMACHD) act as adapters for MMACHC, facilitating the transfer of cobalamin to its final destinations, namely methionine synthase (MS) in the cytosol or its mitochondrial transporter methylmalonic aciduria type A protein (MMAA) [19]. The gene discussed is MTR; the disease is methylmalonic acidemia.