MTHFR and urea cycle disorder: Published literature on MEDLINE was assessed regarding diseases of homocysteine metabolism (DHM; cystathionine beta-synthase deficiency [CbS-D] and homocysteinemia due to methyltetrahydrofolate reductase deficiency [MTHFR-D]), urea cycle disorders (UCD), acute porphyria (POR), Wilson disease (WD), cerebrotendinous-xanthomatosis (CTX) and Niemann-Pick disease type C (NP-C).