Folate deficiency causes anaemia owing to impaired purine and pyrimidine biosynthesis. Vitamin B12 deficiency causes an identical anaemia owing to metabolic trapping of intracellular folate. Vitamin B12 also causes nerve damage. Unlike the anaemia, the nerve damage is probably owing to inhibition of methyltransferase involved in nerve cell biosynthesis. This decreased activity is owing to a reduction in the availability of SAM, the methyl donor, together with product inhibition by its demethylated form, SAH.