MTHFR and homocystinuria: The MTHFR 677TT genotype alone or in combination with the heterozygous 1298AC genotype, causes mild hyperhomocysteinemia in the presence of low folate status, while thrombotic disease seems to be primarily associated with homocystinuria (Brattström & Wilcken, 2000), which could explain the absence attenuation of hemophilia in the MTHFR 677TT carriers.