Perturbations in the activities of enzymes involved in these processes such as methylene tetrahydrofolate reductase (MTHFR), methylene tetrahydrofolate dehydrogenase (MTHFD), methionine synthase reductase (MTRR), cystathionine bsynthase (CBS) may results in altered levels of homocysteine and thus metabolic disorders. The gene discussed is MTRR; the disease is metabolic disease.