MTHFR and homocystinuria: Genetic causes of hyper homocysteinaemia include cystathione beta synthase (CBS) deficiency or ‘classical homocystinuria’ (variably or not pyridoxine responsive) [85, 86], methylene-tetra hydro-folate reductase (MTHFR) deficiency [87] and cobalamin C (Cbl C) disease which results in combined methyl malonic acidaemia and homocystinuria due to deficient adenosyl-cobalamin [88].