Most cases of methionine synthase deficiency are symptomatic within 2 years of birth with many patients rapidly developing severe encephaloptahy.

It is also called Methionine synthase deficiency, Tetrahydrofolate-methyltransferase deficiency syndrome, and N5-methylhomocysteine transferase deficiency.

The disorder may be distinguished from the re-methylation defects (e.g. MTHFR, methionine synthase deficiency and the cobalamin defects) in lieu of the elevated methionine concentration.

Because this process inevitably shuts down all methionine synthase activity, defects or deficiencies in methionine synthase reductase have been implicated in some of the disease associations for methionine synthase deficiency discussed below.

It is also called Methionine synthase deficiency, Tetrahydrofolate-methyltransferase deficiency syndrome, and N5-methylhomocysteine transferase deficiency.