By Russell Browne (Dip. Adv. Nut.) · Specialist in Genetic Nutrition & Methylation Protocols

If you’ve read about MTHFR, you’ve probably met its two headline variants: C677T and A1298C. C677T gets most of the attention, but A1298C comes up constantly in questions — usually alongside worry about carrying “both.” This guide explains what A1298C is, how it differs from its better-known sibling, what the evidence genuinely supports, and how to find your own result.

A quick refresher on MTHFR

The MTHFR gene codes for an enzyme that helps your body process folate (vitamin B9) and, downstream, keep the amino acid homocysteine in balance. When the enzyme works less efficiently, homocysteine can rise — which is the thread running through most MTHFR research. (For the full mechanism, see our C677T guide.)

What makes A1298C different

A1298C (genetic ID rs1801131) is a second common variant in the same gene. The practical difference from C677T is one of degree: A1298C tends to have a smaller effect on enzyme efficiency than C677T does. Carrying two copies is generally associated with a more modest reduction in enzyme activity than two copies of C677T.

As with any variant, you can carry no copies, one copy (heterozygous), or two copies (homozygous) — one inherited from each parent.

The “compound heterozygous” question

The scenario people ask about most is carrying one copy of C677T and one copy of A1298C — often called compound heterozygous. Studies suggest this combination is associated with a moderate reduction in enzyme efficiency, more than a single A1298C copy alone. It’s a genuinely common combination, and — this is the part worth internalizing — being compound heterozygous is not a diagnosis and does not, by itself, mean you have or will develop any condition.

What the research actually shows

Like C677T, A1298C has been studied for associations with relative shifts in risk for various outcomes across populations. The evidence is generally weaker and less consistent than for C677T. The honest summary: A1298C is common, its individual effect is modest, and any association is a matter of adjusted odds across groups — not a personal prediction.

Be especially skeptical of confident online claims tying A1298C to long lists of symptoms. The science doesn’t support that level of certainty, and neither will we.

How to find your A1298C result

Your existing 23andMe or AncestryDNA file already contains this marker — look for rs1801131. As with C677T, raw files may report it on a different strand than research papers use, which is a common source of confusion when reading the file by hand. An analysis tool resolves the strand and reports your genotype in plain terms, which is why most people find that easier than manual lookup.

What your result means — and what to do

If you carry one or two copies of A1298C, or the compound-heterozygous combination:

• It’s common and, on its own, not cause for alarm.
• It may be associated with modest changes in folate processing for some people.
• Whether it warrants any action depends entirely on your individual picture — labs, history, symptoms — which is a discussion for a qualified practitioner, not a protocol from the internet. We deliberately don’t prescribe here.

See your own result

The simplest way to stop wondering is to look. Your existing raw file already contains rs1801131 — no new test required.

Upload your 23andMe or AncestryDNA file to see your MTHFR A1298C result in a plain-language variant report, then review it with your practitioner in context.

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About the Author

Russell Browne (Dip. Adv. Nut.)
Specialist in Genetic Nutrition & Methylation Protocols

Russell Browne is a qualified practitioner of Advanced Nutrition, Advanced Herbalism, and Homeopathy, with a specialized clinical focus on genetic nutrition, methylation pathways, and detoxification. With years of dedicated clinical experience helping patients decode complex chronic health issues, Russell is also the author of How to Treat Your MTHFR Gene Mutations the Right Way – The Genetic Advantage and several works on genetic wellness. He is committed to bridging the gap between complex genetic data and practical, root-cause clinical care.

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