Choline metabolism, including the BHMT gene for betaine–homocysteine S-methyltransferase, the CHDH gene for choline dehydrogenase, the MTHFD1 gene for methylenetetrahydrofolate dehydrogenase 1, and the PEMT gene for phosphatidylethanolamine N-methyltransferase. B12, vitamin B-12; BHMT, betaine-homocysteine S-methyltransferase; CHDH, choline dehydrogenase; MTHFD1, methylenetetrahydrofolate dehydrogenase 1; PEMT, phosphatidylethanolamine N-methyltransferase; THF, tetrahydrofolate. Reproduced with permission from reference 2.