MTRR A66G: your B12 recycling, decoded
MTHFR gets all the press. Its partner enzyme, MTRR, does the other half of the methylation work and almost nobody talks about it.
Methylation is the cycle that clears homocysteine and tags your DNA, and it only runs if vitamin B12 stays in its active form. MTRR is the enzyme that keeps regenerating that active B12. A common variant called A66G makes the regenerating a bit sloppier, which can let homocysteine creep up when your B12 is already running low.
What MTRR does
Be honest about the B12 piece: it's general advice, not something special to your DNA. The variant doesn't change how you respond to B12. It just raises the payoff for keeping your B12 topped up. The form that absorbs well and is already active is methylcobalamin.
It travels with MTHFR. If you carry both variants, it makes more sense to feed the whole cycle (methylfolate plus B12) than to fixate on a single nutrient.
Your variants, decoded
The G allele goes with slower B12 regeneration.
| AA | Normal recycling speed. |
| AG | One copy. A modest dip. |
| GG | Two copies. The slowest of the three, so your B12 status matters most here. |
Genotypes are shown order-insensitively and on the forward strand; your own export may print the complementary letters — the meaning is the same.
What the research suggests
Your MTRR A66G variant slows how well you regenerate active vitamin B12 in the methylation cycle, which can nudge homocysteine up. Worth being clear: the B12 benefit is general, not unique to your genotype. Your variant just raises the value of keeping B12 topped up in the active methylcobalamin form.
PubMed 37917901 · MTRR A66G & B12 / homocysteine status
Educational only — not medical advice. “General evidence” means the finding is real but the supplement’s benefit isn’t unique to your genotype.
See this matched to your own DNA — free.
Upload your 23andMe or AncestryDNA file and get your actual MTRR result, plus every other actionable variant — each line cited, your file never stored.
Questions
What does the MTRR A66G variant do?
It makes your body slower at recycling vitamin B12 inside the methylation cycle, which can push homocysteine up when B12 is low. So it's worth keeping your B12 in good shape.