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Causes of severe hyperhomocysteinemia and serum abnormalities
| Disease | Enzyme/cofactor | Hcy | Met | MMA |
| Classical homocystinuria | Cystathionine β-synthase | ↑ | ↑ | N |
| Cobalamin deficiency | Cobalamin | ↑ | ↓ | ↑ |
| MTHFR deficiency | MTHFR | ↑ | N/↓ | N |
Hcy, homocysteine; Met, methionine; MMA, methylmalonic acid; MTHFR, methilenetetrahydrofolate reductase.
