MTHFR and homocystinuria: Recommended testing includes a very long chain fatty acid (VLCFA) profile in men (ALD), specific white cell enzyme activities (Krabbe disease, MLD), raised serum cholestanol/urinary bile alcohols (cerebrotendinous xanthomatosis (CTX)) and plasma amino acids (methylenetetrahydrofolate reductase (MTHFR) deficiency and homocystinuria) (table 1).