Supports methionine synthase
Methionine synthase can become oxidatively inactive. MTRR participates in the reductive process required to restore the enzyme to a functional state.
MTRR methylation gene guide
MTRR Gene and Methylation: B12, MTR & A66G Explained
The MTRR gene encodes methionine synthase reductase, an enzyme that helps restore active methionine synthase so the folate- and vitamin-B12-dependent remethylation of homocysteine can continue.
MTRR is closely connected with MTR, MTHFR and the wider methylation gene network. Its role is more precise than simply “recycling B12,” and an MTRR genetic result should not be treated as a direct measurement of vitamin B12, folate, homocysteine or overall methylation status.
- Encodes methionine synthase reductase
- Helps reactivate methionine synthase
- Supports the folate/B12 remethylation pathway
- A66G rs1801394 is a commonly studied variant
Full name
Methionine synthase reductase.
Main partner
MTRR helps maintain functional methionine synthase.
B12 connection
MTR is a vitamin-B12-dependent enzyme.
Common variant
A66G is also known as rs1801394.
MTRR explained
What Does the MTRR Gene Do?
MTRR provides instructions for making methionine synthase reductase. Its key role is to help restore methionine synthase after that enzyme becomes inactive during normal catalytic cycling.
Connects with vitamin B12
Methionine synthase depends on a cobalamin, or vitamin B12, cofactor. MTRR therefore has an important indirect relationship with B12-dependent methionine synthase activity.
Supports homocysteine remethylation
By helping maintain functional methionine synthase, MTRR supports the pathway that converts homocysteine back to methionine.
Quick answer: MTRR does not simply “recycle vitamin B12.” It helps reactivate vitamin-B12-dependent methionine synthase, allowing that enzyme to continue transferring a methyl group during the conversion of homocysteine to methionine.
MTRR and MTR
How Do MTRR and MTR Work Together?
MTR encodes methionine synthase. That enzyme uses methylcobalamin as an intermediate carrier while transferring a methyl group from 5-methyltetrahydrofolate to homocysteine.
During normal enzyme activity, the cobalamin cofactor can become oxidized, leaving methionine synthase inactive. MTRR helps support the reductive reactivation process required to restore functional methionine synthase.
This MTR-MTRR partnership is why both genes commonly appear together in methylation and one-carbon metabolism reports.
Pathway context
Where MTRR Fits in the Folate and B12 Methylation Pathway
MTRR makes more sense when it is viewed as one part of the folate/B12-dependent remethylation pathway rather than as a stand-alone “methylation gene.”
5-Methyl-THF
Provides a methyl group to the methionine synthase reaction.
Vitamin B12
Acts as an intermediate methyl carrier within methionine synthase.
MTR + MTRR
MTR performs the reaction while MTRR helps restore active MTR.
Homocysteine
Receives the methyl group during remethylation.
Methionine
Returns to the wider methionine and methylation cycle.
Vitamin B12 context
Does MTRR “Recycle Vitamin B12”?
That phrase is often used as shorthand, but it can be misleading. MTRR's better-defined role is helping reactivate methionine synthase, whose catalytic process depends on a cobalamin cofactor.
MTR uses cobalamin
Methionine synthase uses a vitamin-B12-derived cobalamin cofactor as an intermediate methyl carrier during homocysteine remethylation.
MTR can become inactive
Oxidation of the cobalamin cofactor can temporarily inactivate methionine synthase during normal enzyme cycling.
MTRR helps restore activity
MTRR participates in the reductive reactivation system that allows methionine synthase to return to its functional state.
Important:
An MTRR genotype does not directly measure serum vitamin B12,
methylmalonic acid, folate, homocysteine or functional vitamin status.
Those are different types of measurements.
Common MTRR variant
What Is MTRR A66G, or rs1801394?
MTRR A66G, identified as rs1801394, is one of the most commonly researched MTRR polymorphisms. It is frequently included in studies of folate, vitamin B12 and one-carbon metabolism.
Research has examined possible relationships between this variant and different biochemical or health outcomes. Results are not consistent enough to use an A66G genotype alone as a direct prediction of homocysteine, folate or vitamin B12 status.
It is inherited DNA information
rs1801394 identifies a genetic difference in MTRR. A genotype is generally stable and is not a real-time biochemical measurement.
Research findings vary
Studies have investigated A66G in many populations and clinical contexts, but an association in one population should not be treated as a universal individual prediction.
It needs pathway context
MTRR findings are more meaningful when considered alongside MTR, folate-related genes, other metabolic factors and appropriate clinical information.
Genetic interpretation
What Can an MTRR Genetic Result Actually Tell You?
An MTRR result can identify inherited variation when that marker is included in the test. It cannot substitute for current laboratory measurements.
What an MTRR result can show
- Which tested MTRR genetic variant or genotype you inherited.
- Whether a marker such as rs1801394 is included in your results.
- Biological context about methionine synthase reactivation.
- A genetic data point that can be considered alongside other methylation-related genes.
What an MTRR result cannot establish
- Your current vitamin B12 or folate concentration.
- Your current blood homocysteine level.
- The exact activity of methionine synthase in your body today.
- Whether you have a vitamin deficiency.
- Which form or dose of vitamin B12, folate or other supplement you should take.
Bottom line: MTRR genetics can add pathway context, but genotype and current nutrient status are different questions. If B12, folate or homocysteine is clinically important, appropriate laboratory assessment provides different information from a DNA result.
Broader pathway interpretation
Why MTRR Should Not Be Interpreted by Itself
Homocysteine remethylation depends on several connected nutrients, enzymes and pathways. Looking at MTRR alone can make the biology appear much simpler than it actually is.
- Review MTR. MTRR's primary function is closely tied to maintaining functional methionine synthase.
- Consider folate metabolism. The MTR reaction depends on a methyl group ultimately supplied through folate metabolism.
- Consider the B12-dependent step. Methionine synthase uses a cobalamin cofactor during the reaction.
- Recognize alternate remethylation routes. BHMT provides another biological route for homocysteine remethylation using betaine.
- Separate genetics from current status. Diet, medications, health conditions and laboratory values can provide information a genotype cannot.
Related methylation genes
Genes Commonly Reviewed Alongside MTRR
MTRR is part of a connected one-carbon metabolism network. Reviewing related genes can provide better pathway context than focusing on one polymorphism alone.
MTR
Encodes methionine synthase, the B12-dependent enzyme that MTRR helps reactivate.
MTHFR
Participates in folate metabolism and supports production of 5-methyltetrahydrofolate used by methionine synthase.
BHMT
Provides an alternative homocysteine-remethylation route using betaine rather than the folate/B12-dependent pathway.
COMT
Uses SAM as a methyl-group donor in catechol metabolism and provides another view of how methyl groups are used downstream.
MTRR genetic testing
What to Check Before Choosing a Test for MTRR
If MTRR is one reason you are exploring genetic testing, confirm the exact genes and variants included rather than assuming all methylation tests provide the same coverage.
Confirm the MTRR variant
Check whether the report includes rs1801394 or another MTRR marker and understand exactly what the provider reports.
Look beyond one SNP
A useful report should provide context for related genes and pathways rather than assigning a complete interpretation to one polymorphism.
Preview the report
Review the sample methylation report before ordering so you can see how variants, pathways and limitations are explained.
Testing in the United States
Considering MTRR Genetic Testing in the USA
MTRR biology does not change by state, so geographic relevance should focus on the real testing experience: current availability, sample collection, shipping, return instructions and report access.
Before ordering, review U.S. shipping information, testing locations and how the testing process works.
Verify current availability
Confirm that the test can currently be ordered and shipped to your location before purchasing.
Review sample collection
Understand the required sample, preparation steps and return workflow before starting.
Know what is being measured
A DNA genotype is different from laboratory measurements of vitamin B12, folate, methylmalonic acid or homocysteine.
Scientific context & EEAT
Evidence Used to Explain MTRR
MTRR interpretation should distinguish established enzyme biology from association studies involving common polymorphisms.
NCBI Gene — MTRR
Authoritative gene record for human methionine synthase reductase.
Original MTRR cloning and functional research
Describes methionine synthase inactivation and its reductive
reactivation pathway.
Research on MTRR A66G and homocysteine
Illustrates why a common MTRR polymorphism should not automatically
be treated as a predictor of an individual's homocysteine response.
Evidence standard:
Common genetic association studies can generate useful research
questions, but an association observed in a population does not mean a
particular genotype determines an individual's nutrient status,
laboratory values or clinical outcome.
MTRR methylation FAQ
Frequently Asked Questions About the MTRR Gene
Clear answers about MTRR, methionine synthase, vitamin B12, homocysteine, A66G and methylation-related genetic testing.
What does MTRR stand for?
MTRR stands for methionine synthase reductase. The MTRR gene provides instructions for making an enzyme that helps restore functional methionine synthase.
What does MTRR do in methylation?
MTRR helps reactivate methionine synthase after oxidative inactivation. Methionine synthase participates in the folate/B12-dependent remethylation of homocysteine to methionine.
Does MTRR recycle vitamin B12?
That phrase is an oversimplification. MTRR more specifically supports reductive reactivation of methionine synthase, whose catalytic process depends on a cobalamin cofactor.
What is MTRR A66G?
MTRR A66G is a commonly studied genetic polymorphism identified as rs1801394. Research has examined it in relation to one-carbon metabolism and many health outcomes, but it should not be interpreted as a stand-alone diagnosis or biochemical test.
Does an MTRR variant mean my vitamin B12 is low?
No. A genetic result does not directly measure current vitamin B12 concentration or functional vitamin B12 status. Those questions require appropriate clinical assessment.
Can an MTRR variant tell me my homocysteine level?
No. Your current homocysteine level is a biochemical measurement and cannot be determined directly from an MTRR genotype.
What is the difference between MTR and MTRR?
MTR encodes methionine synthase, which carries out the remethylation reaction. MTRR encodes methionine synthase reductase, which helps restore methionine synthase after it becomes inactive.
Should MTRR be interpreted with MTHFR?
They participate in connected one-carbon metabolism pathways, so broader pathway context can be more informative than focusing on either gene alone. Their functions are different and each result still has interpretation limits.
Can MTRR genetics tell me what supplements to take?
No. An MTRR result alone should not determine vitamin B12, folate or other supplement choices or doses. Those decisions require broader health and clinical context.
Important health and interpretation information:
MTRR genetic information is educational. A genotype does not directly
measure vitamin B12, folate, methylmalonic acid, homocysteine,
methionine synthase activity or current methylation status. Genetic
results should not be used by themselves to diagnose a deficiency or
determine supplements, medication or treatment. Discuss medical
concerns and clinical testing with a qualified healthcare professional.
Scientific context reviewed August 20, 2026. Gene interpretation and testing specifications should be rechecked as scientific evidence and products evolve.
Explore broader methylation genetics
Go Beyond a Single MTRR Gene Result
MTRR is closely connected with methionine synthase, folate, vitamin B12 and homocysteine remethylation, but one genetic variant cannot describe your entire methylation pathway or current nutrient status.
Understand the pathway first, then compare the testing experience.