Homocystinuria due to MTHFR deficiency is a genetic condition that results from poor metabolism of folate (also called vitamin B9), due to a lack of working enzyme called MTHFR. The gene that tells our body how to make the enzyme is also called MTHFR. At least 40 rare MTHFR gene variants have been found in people with decreased or no working enzyme. Very common gene variants (C677T and A1298C) can cause some decrease in enzyme function. People with Homocystinuria due to MTHFR deficiency tend to have two rare variants or sometimes a rare variant and a common variant. Very rarely people inherit a combination of three or four common variants from their parents (for example two C677T variants and two A1298C variants) and may also develop very high levels of homocystine in their body. Homocystinuria usually does not show symptoms in a newborn baby. Symptoms may include abnormal clotting, developmental delay, seizures, intellectual disability, and microcephaly. Severe homocystinuria can also be caused by gene variants in other genes besides MTHFR.