The MTR gene provides instructions for making an enzyme called methionine synthase. This enzyme plays a role in processing amino acids, the building blocks of proteins. Specifically, methionine synthase carries out a chemical reaction that converts the amino acid homocysteine to another amino acid called methionine.
Your body uses methionine to make proteins and other important compounds. To function properly, methionine synthase requires methylcobalamin (a form of vitamin B12) and another enzyme called methionine synthase reductase, which is produced from the MTRR gene.
The A allele is associated with reduced activity and therefore potentially the buildup of homocysteine, especially when present in association with other mutations in methylation cycle enzymes MTHFR, MTRR, and SHMT1.