MTHFR and homocystinuria: The most common diagnosis of type of homocystinuria at baseline was B6 non-responsive CBS deficiency (n = 49, 39.2%), followed by Cbl metabolism defects (n = 45, 36.0%), MTHFR deficiency (n = 21, 16.8%), and B6 responsive CBS deficiency (n = 9, 7.2%; Table 1).