rs1801133 MTHFR
MTHFR C677T homozygous. ~70% reduced enzyme activity. Higher folate/methylfolate needs. Elevated homocysteine risk.
What is rs1801133?
rs1801133 is a single-nucleotide polymorphism (SNP) in the MTHFR gene (Methylenetetrahydrofolate reductase). Each person carries two copies, one from each parent, so a result is reported as a pair of letters such as TT. The table below describes what studies associate with each genotype; the association is statistical, across populations, and is not a diagnosis.
What does each rs1801133 genotype mean?
| Genotype | Association | What the evidence says | Confidence |
|---|---|---|---|
| TT | increased | MTHFR C677T homozygous. ~70% reduced enzyme activity. Higher folate/methylfolate needs. Elevated homocysteine risk. | 92% |
| CT / TC | increased | MTHFR C677T heterozygous. ~35% reduced enzyme activity. May benefit from methylfolate. | 88% |
| CC | typical | Normal MTHFR function. Standard folate metabolism. | 90% |
How common is the T allele of rs1801133?
Minor-allele frequency by population (gnomAD / 1000 Genomes):
| European | 33.0% |
|---|---|
| East Asian | 30.0% |
| African | 10.0% |
| South Asian | 15.0% |
| Admixed American | 40.0% |
| Global | 25.0% |
Which drugs have annotations for rs1801133?
PharmGKB clinical annotations (level 1A is the strongest evidence, 4 the weakest):
| Drug | Annotation type | Evidence level |
|---|---|---|
| methotrexate | Toxicity, Efficacy, Metabolism/PK, Dosage | 2A |
| antipsychotics | Toxicity, Efficacy, Metabolism/PK, Dosage | 2A |
| nitrous oxide | Toxicity, Efficacy | 2A |
Is rs1801133 on 23andMe and AncestryDNA chips?
Yes — it is genotyped directly on 23andMe V3, 23andMe V4, 23andMe V5, AncestryDNA V1, AncestryDNA V2. Orviva reads it from a raw-data export without imputation.
Sources
Upload your 23andMe, AncestryDNA or WeGene raw file → · Ask G2: “What is my genotype at rs1801133 and what does it mean for me?”
Genetic associations describe risk across populations, not a diagnosis for any one person. Most variants shift risk modestly and act alongside lifestyle, environment and other genes. Consumer DNA chips do not read every position and can be wrong at any single one. For general education only; talk to a clinician or genetic counsellor before acting on anything here, and never change a prescribed medicine without your prescriber.