rs1801133
MTHFR C677T, and why it is oversold
What this position does
This is the single most over-interpreted position in consumer genetics. It genuinely does reduce the activity of an enzyme in folate metabolism. What it does not do is most of what the internet says it does.
What each result means
| If you carry | What it means |
|---|---|
| No copies of the effect version GG | You do not carry C677T Two copies of the higher-activity version. |
| One copy AG | You carry one copy, which changes nothing in practice Carried by roughly a third of people. Enzyme activity is modestly reduced, with no established consequence for anyone eating an ordinary diet. |
| Two copies AA | You carry two copies, which is common and still changes little Carried by roughly one person in ten of European descent, and more in some Mediterranean and Hispanic populations. Enzyme activity is around thirty per cent lower and homocysteine runs slightly higher, particularly on a low-folate diet. Large studies have not found the clotting, miscarriage or cardiac effects widely attributed to it, and major genetics bodies have recommended against testing for it. |
Worth knowing. It is on this list precisely so a positive result can be put in proportion. If a practitioner proposes an expensive supplement protocol on the strength of it, the cited guideline is the thing to read first.
How common each version is
| Ancestry group | A (the effect version) | |
|---|---|---|
| African | 9.0% | |
| American | 47.4% | |
| East Asian | 29.6% | |
| European | 36.5% | |
| South Asian | 11.9% |
1000 Genomes Project phase 3, via Ensembl, retrieved 2026-09-23. These groupings are coarse and flatten a great deal of internal diversity, particularly within Africa. A variant being common somewhere tells you where a piece of your ancestry probably comes from, and nothing else.
How many people carry each result
| Ancestry group | GG | AG | AA |
|---|---|---|---|
| African | 549 / 661 | 105 / 661 | 7 / 661 |
| European | 204 / 503 | 231 / 503 | 68 / 503 |
| East Asian | 259 / 504 | 192 / 504 | 53 / 504 |
| South Asian | 377 / 489 | 108 / 489 | 4 / 489 |
| American | 91 / 347 | 183 / 347 | 73 / 347 |
Counts of people, not modelled frequencies: every one of the 2,504 individuals sequenced by the 1000 Genomes Project phase 3, tallied by the genotype they carry. Deriving these from allele frequencies instead would assume random mating inside each group and undercount the homozygotes.
How to find out which you have
If you have taken a test with 23andMe, AncestryDNA, MyHeritage, Family Tree DNA, TellMeGen or Living DNA, this position is very likely already in the raw data file they gave you. You do not need another test.
One thing to watch if you look it up by hand: testing companies report some positions from one DNA strand and some from the other, and the file does not say which. Read the letters as they appear and you will get the opposite of the truth for some positions. Sequencings resolves that automatically.
Source
Coordinates and allele pair verified against Ensembl GRCh37. Direction of effect verified against VEP protein consequences or 1000 Genomes population frequency. Category: Health-adjacent.
Related
- rs1801131 — MTHFR A1298C
Where this fits
Common questions
What is rs1801133?
rs1801133 is a position in the MTHFR gene on chromosome 1. This is the single most over-interpreted position in consumer genetics. It genuinely does reduce the activity of an enzyme in folate metabolism. What it does not do is most of what the internet says it does.
What does it mean if I have AA at rs1801133?
You carry two copies, which is common and still changes little. Carried by roughly one person in ten of European descent, and more in some Mediterranean and Hispanic populations. Enzyme activity is around thirty per cent lower and homocysteine runs slightly higher, particularly on a low-folate diet. Large studies have not found the clotting, miscarriage or cardiac effects widely attributed to it, and major genetics bodies have recommended against testing for it.
What does it mean if I have GG at rs1801133?
You do not carry C677T. Two copies of the higher-activity version.
How do I find out my rs1801133 genotype?
If you have taken a home DNA test, this position is very likely in the raw data file you were given. Sequencings reads that file in your browser without uploading it, and resolves which DNA strand each position was reported from, which is the step most manual lookups get wrong.
This is reference information, not medical advice, and not a diagnosis. See the medical disclaimer. Category: Health-adjacent.