Hence, deficiency of either vitamin leads to elevated serum homocysteine.43 Vitamin B12 deficiency also induces a secondary, functional folate deficiency, known as a “5‐methyltetrahydrofolate trap.” Because methionine synthase is the only enzyme that metabolizes 5‐methyltetrahydrofolate, vitamin B12 deficiency results in its accumulation, thus “starving” other folate‐dependent metabolic pathways for folate cofactors, including nucleotide biosynthesis. The gene discussed is MTR; the disease is vitamin B12 deficiency.