An amino acid metabolic disorder characterized by failure of cells to incorporate methyltetrahydrofolate into methionine, impaired methionine synthase activity in the presence of a reducing agent, and somewhat variable features that include delayed psychomotor development, hypotonia, megaloblastic anemia, homocystinuria, and hypomethioninemia that has_material_basis_in homozygous or compound heterozygous mutation in MTR on chromosome 1q43.