MTHFR

What it isA gene encoding the enzyme that converts 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the active form of folate used in methylation. Two common polymorphisms reduce enzyme activity: C677T (homozygous TT cuts activity roughly 70%) and A1298C (milder effect).

DoesDetermines how efficiently dietary folate is converted to its active methylated form, affecting homocysteine metabolism and DNA methylation.

GoodWild-type genotype gives full enzyme activity and unconstrained folate metabolism.

BadReduced-activity variants modestly raise homocysteine when folate intake is low, with weak associations to cardiovascular risk, neural tube defects, and pregnancy complications. The clinical importance is often overstated; most carriers do fine with adequate folate intake.