Overview of mammalian one carbon metabolism. Four genetic mouse models involving disruptions of enzymes in this pathway have been highlighted in grey as has betaine-homocysteine methyltransferase, where transient enzyme deficiencies have been induced by treatment of mice with an inhibitor. Abbreviations: BHMT, betaine-homocysteine methyltransferase; CBS, cystathionine β-synthase; CGL, cystathionine γ-lyase; DHFR, dihydrofolate reductase; DMGD, dimethylglycine dehydrogenase; GNMT, glycine N-methyltransferase; MAT, methionine adenosyltransferase; MS, cobalamin-dependent methionine synthase; MSR, methionine synthase reductase; MTHFR, methylenetetrahydrofolate reductase; MTs, AdoMet-dependent methyltransferases; SAHH, S-adenosylhomocysteine hydrolase; SDH, sarcosine dehydrogenase; SHMT, serine hydroxylmethyltransferase; TS, thymidylate synthase.