CBS and hyperinsulinemic hypoglycemia, familial, 4: Folate (vitamin b9) and cobalamin (vitamin b12) deficiency can be due to insufficient dietary intake and/or mutation in genes involved in the folate cycle, namely methylenetetrahydrofolate (MTHF) reductase gene, transcobalamin 2 (TCN2), and cystathionine beta-synthase (CBS) [106].