Why MTHFR C677T Can Be Hard to Find in Genome Explorer
By Sequencing Team, The team of bioinformaticians, Genetic Health Coaches, and writers at Sequencing.
Searching for MTHFR C677T and finding no exact match can make it look as though the variant is missing from your DNA data. Often, the issue is not the sequencing result. It is the name used to describe the variant.
A single variant can have several valid representations depending on whether it is described by a database identifier, a genomic coordinate, a transcript, or a protein change. Sequencing applications are built around current variant data, while many laboratory reports and articles use historical, transcript-oriented names.
For MTHFR C677T, those naming systems are frequently different.
One Variant, Several Names
The variant commonly called MTHFR C677T is represented by ID rs1801133. The following descriptions all refer to the same single-nucleotide variant at this position:
- Common historical name: MTHFR C677T or 677C>T
- dbSNP identifier: rs1801133
- Current transcript: NM_005957.5:c.665C>T
- Protein change: NP_005948.3:p.Ala222Val or A222V
- GRCh38 genomic HGVS: NC_000001.11:g.11796321G>A
- GRCh37 genomic HGVS: NC_000001.10:g.11856378G>A
This is why a search for “C677T” may not return the result you expect. That familiar label is a historical name; it is not the current description for the variant.
The NCBI ClinVar record for Variation 3520 links these names to the same variant.
Why C>T Becomes G>A
At first glance, C>T and G>A appear to describe different DNA changes. They are actually complementary descriptions of the same change viewed from opposite DNA strands.
MTHFR is read from the opposite strand of the chromosome reference sequence, which is known as reverse orientation. DNA bases pair predictably:
- C pairs with G
- T pairs with A
Therefore, the transcript-oriented C>T change corresponds to the genomic forward-strand G>A change.
Genome Explorer displays variants in the orientation of the GRCh38 chromosome reference. For rs1801133, that means chromosome 1, position 11,796,321, with genomic alleles G and A.
Why the Genotype May Also Look Different
The same strand difference affects genotype as well. A result written using the historical C677T convention may not use the same letters shown in Genome Explorer.
- Genome Explorer genomic genotype GG is the historical C677T genotype 677CC.
- Genome Explorer genomic genotype GA is the historical C677T genotype 677CT.
- Genome Explorer genomic genotype AA is the historical C677T genotype 677TT.
For example, genomic AA and historical 677TT are the same diploid genotype represented in opposite orientations. The letters differ, but the underlying DNA call does not.
This is an important detail when comparing results across platforms. Matching only the displayed genotype letters, without checking strand and variant identity, can make concordant results appear inconsistent.
How to Find MTHFR C677T in Your Data
In Genome Explorer, the most reliable search term is the stable dbSNP identifier: rs1801133
If you search by genomic position instead, use the coordinate for the GRCh38 build:
- GRCh38: chromosome 1, position 11796321
When reviewing the result, confirm all of the following:
- Variant ID: rs1801133
- Gene: MTHFR
- Chromosome and position: chr1:11796321 on GRCh38
- Genomic alleles: Ref: G, Alt: A
- Genotype possibilities: GG, GA, or AA in the genomic display
A Missing Name Is Not the Same as Missing Data
This case illustrates a broader challenge in genomics: variant names are not universal labels. The same DNA change can be represented at the genomic, transcript, and protein levels, and each reference sequence has its own numbering system.
For MTHFR C677T, the most dependable bridge between those systems is the rs1801133 identifier.
Not seeing the text “C677T” does not, by itself, show that the site was not sequenced or called. It may simply mean that the application is displaying the same variant using a more precise genomic representation. Coverage, data source, and active filters can also affect what is visible, so those should be checked separately if neither the rsID nor the correct coordinate returns a result.
This article addresses variant identification only. Finding a variant is separate from interpreting its clinical significance or deciding whether any health action is appropriate.
Key Takeaways
- MTHFR C677T and rs1801133 refer to the same variant.
- The current transcript name is NM_005957.5:c.665C>T, or C665T.
- Genome Explorer uses the GRCh38 genomic representation NC_000001.11:g.11796321G>A.
- C>T becomes G>A because transcript and genomic descriptions use opposite strand orientations.
- Historical 677TT corresponds to genomic AA, 677CT corresponds to GA, and 677CC corresponds to GG.
- Search by rs1801133 first, then confirm the reference build, position, alleles, and genotype orientation.
Related Reading
- MTHFR Gene: Variants, Testing and What Your DNA Can Tell You: the guide this article belongs to, covering the two common variants and what medical guidelines say about testing.
- MTHFR Thermolabile Polymorphism (C677T): the common variant in more detail.
Get Tested With Whole Genome Sequencing
Finding a variant is separate from deciding what it means. Sequencing reads 100% of your DNA, including over 30,000 genes, so MTHFR is part of your data. Sequencing's Kit Bundles include a Whole Genome Sequencing kit and Next Gen Disease Screening, which analyzes your genome for more than 15,000 conditions. Guidelines advise against relying on a common MTHFR variant alone, so bring your results to a clinician.