The methyl (-CH3) donor choline and the methyl carriers folate and vitamin B12 (cobalamin) all contribute to the metabolic conversion (or methylation) of homocysteine to methionine. Methyl-folate can methylate homocysteine only by transferring its methyl group to vitamin B12 (now called methylcobalamin). If there isn't adequate B12 in the cells, then folate can't do its job and conversion of homocysteine to methionine will rely more on choline. Similarly, if there is insufficient choline available, the reaction will rely more on folate and B12. In other words, there is a reciprocal inter-dependance between these three essential nutrients of the methionine pathway. This is also the reason that the build up in blood of homocysteine is an accurate indication of deficiency of choline, folate and B12, or all three.