With few exceptions, the pathological consequences of vitamin B12 deficiency have been based on the only two known vitamin B12 dependent biochemical reactions in the mammalian cells, involving the mitochondrial L-methylmalonyl-coenzyme A mutase (MMCM; EC 5.4.99.2) and the cytoplasm homocysteine (Hcy) methyltransferase, also referred as methionine synthase (MS; EC 2.1.1.13) [2]. This evidence concerns the gene MTR and vitamin B12 deficiency.