Vitamin B12 is a cofactor of methionine synthase and methylmalonyl-CoA mutase.[1] Its deficiency contributes to reduced methionine synthase activity, which in turn generates myelin synthesis disorder as SCD, in which the thickest myelin sheaths (such as posterior and pyramidal lateral columns) are preferentially affected.[5] Vitamin B12 deficiency also results in macrocytic anemia (PA in this case) which is associated with decreased thymine synthesis. The gene discussed is MTR; the disease is macrocytic anemia.