What MTRR A66G Means in Your 23andMe Raw Data

MTRR A66G is rs1801394, a common variant in the gene for methionine synthase reductase. In a 23andMe raw file you will usually see it as A/A, A/G or G/G. The G allele is the one most methylation panels flag, and G/G is what they usually call +/+.

In plain terms, this SNP changes the enzyme that recycles vitamin B12 inside the methylation cycle. It is not a diagnosis. It is not a reason by itself to start high-dose B12, methylfolate or anything else.

What does MTRR A66G mean in my 23andMe raw data?

If you search a 23andMe raw file for rs1801394, you get one line with a genotype. Most files report one of:

• A/A
• A/G
• G/G

The A allele is the reference allele in dbSNP, but frequency varies a lot by population. Methylation panels, including Genetic Genie, usually treat G as the variant allele at this marker. So:

The key thing people get wrong is treating G/G as a broken enzyme. MTRR A66G is a mild missense change, swapping isoleucine for methionine at codon 22. Studies suggest a small average effect on MTRR activity, often only when B12 or B2 status is also low. Many people with G/G never have a high homocysteine level and never feel any effect.

One genotype at MTRR A66G is a hint, not a verdict. The thing worth checking is whether your homocysteine and B12 levels are actually moving.

Why the enzyme matters: the B12 recycling loop

MTRR stands for methionine synthase reductase. The enzyme in the same pathway, methionine synthase (MTR), converts homocysteine into methionine. During that reaction, the B12 helper attached to MTR gets oxidised and stops working. MTRR recycles that B12 back to its active form.

This is the loop:

When MTRR activity is lower on average, the backup is usually small. Your body keeps running the cycle, but studies suggest a small average effect on homocysteine. The system becomes less flexible mainly if riboflavin (B2) or B12 intake is low.

Methionine is used for methylation, creatine, DNA repair and protein building. Homocysteine is a middle product that should be low in a healthy fasting blood test. That is the number most worth looking at.

What the risk looks like in real life

In most people, MTRR A66G alone does not cause symptoms. When it does show up in studies, it usually appears as a small average shift in homocysteine, not a disease. The effect may be larger in people with low B2 or B12, and it can stack with other methylation variants such as MTHFR C677T.

What you would actually look for on a lab report:

• Fasting homocysteine: many labs call 5 to 15 μmol/L normal, but some clinicians prefer under 10.
• Serum B12: below 250 pg/mL or 185 pmol/L is generally low or marginal in many labs.
• Folate: low-normal folate with a methylation SNP can make the picture more strained.
• Riboflavin status is harder to measure, but low B2 is the relevant cofactor for this exact enzyme.

That is the practical reading. A genotype is static. Homocysteine and B12 are dynamic and can be checked.

Why Genetic Genie flags it as +/+

Genetic Genie reports MTRR A66G as one of its methylation genes. The notation +/- or +/+ refers to the number of G alleles. It is not a diagnosis. It is a shorthand for "you inherited one or two copies of the variant allele at this known common polymorphism."

The panel is useful because it makes you stop and look at the methylation cycle. The panel is dangerous when people read +/+ as "I must treat myself." The common mistake is starting high-dose methyl-B12 or methylfolate immediately after seeing a +/+ result. Some people report anxiety, insomnia, a wired feeling, or a crash after high-dose methylated B vitamins. Methyl-B12 is a form of vitamin B12, while methylfolate is a form of folate, so they are not interchangeable.

The better sequence is: genotype first, then blood work, then targeted change. If your homocysteine is normal and B12 is normal, the genotype is not doing enough to justify high-dose supplements.

What to do next instead of guessing

If you see MTRR A66G in your raw data, the reasonable follow-up is:

• Check fasting homocysteine, serum B12, folate and, if possible, B2 status.
• Look at MTHFR C677T in your file too, because the two often get discussed together.
• Fix diet basics before supplements: animal foods for B12, leafy greens for folate, dairy, eggs or greens for B2.
• If a supplement is considered, start low and adjust based on labs or symptoms, not on the genotype alone.

And a specific caveat: if you are already taking high-dose methylated B vitamins and feel worse, that is worth mentioning to a clinician. The answer is sometimes to stop the supplement, not to raise the dose.

The free HealthOS DNA report does not currently cover MTRR A66G. It does report MTHFR from a raw file. So if you upload a 23andMe file, do not expect MTRR on the report page; it is not in the current gene list.

healthosx.com/dna

The download steps for 23andMe, on the page itself.

healthosx.com/dna

The short version: what is worth acting on, first.

If you want to see what your raw 23andMe file does contain, the free DNA tool at https://healthosx.com/dna reads a 23andMe, AncestryDNA, MyHeritage or whole-genome VCF file in the browser. It reports traits, medication response and carrier-relevant findings, with an optional ancestry breakdown. Whole-genome VCF files without rsID annotations cannot produce the health findings. MTRR A66G is not one of the current single-variant findings, but the report is useful for seeing the evidence on the variants it can call.