MTHFR mutation: a genetic change in the gene that codes for the enzyme methylenetetrahydrofolate reductase. This enzyme is involved in breaking down the amino acid homocysteine. Having a defect in this enzyme leads to elevated blood and urine levels of homocysteine.
There are two common MTHFR mutations, known as C677T and A1298C. About 1/3 of Americans have one copy of the C677T mutation, but having one copy of C677T or one copy of A1298C is not associated with significant health risks. Likewise, having two copies of A1298C is not associated with significant health risks. However, people with two copies of the C677T mutation (about 11% of people in the US), as well as people who have one copy of both the mutations, have an increased risk for cardiovascular conditions including coronary artery disease, blood clots, and stroke.