Methionine synthase (MTR) catalyses the transfer of a methyl group from 5-methyltetrahydrofolate (MTHF) to homocysteine (HCYS) to then form methionine (L-Met). In the first step, MTR mediates the transfer of a methyl group from 5-methyltetrahydrofolate (MTHF) to cob(I)alamin (B12s, bound to the enzyme MTR) to form the cofactor methylcobalamin (MeCbl), the form that activates MTR (Leclerc et al. 1996). Defects in MTR cause methylcobalamin deficiency type G (cblG, methionine synthase deficiency; MIM:250940), an autosomal recessive inherited disease that causes mental retardation, macrocytic anemia, and homocystinuria (Leclerc et al. 1996).