Consequently, this fact makes doctors hard to determine the onset age of homocystinuria due to MTHFR deficiency for patients (a chance of misdiagnosis) and dismiss the link with possible gene variations in MTHFR. Although a lot of examinations and clinical treatments have been conducted for children, unfortunately, their homocysteine and methionine have not been tested, and the opportunity for diagnosis and symptomatic treatment may be missed. Here, MTHFR is linked to homocystinuria.