(6R,S)-5,6,7,8-Tetrahydrofolic acid (THFA), the reduced form of folic acid, serves as a cofactor in methyltransferase reactions and is the major one-carbon carrier in one carbon metabolism. Its metabolites participate in the synthesis of thymidine for incorporation into DNA or the synthesis of purines, as well as in the formation of methionine, which can be converted to the ubiquitous methyl donor, S-adenosylmethionine . THFA has also been used to study the activation of metabolite-binding riboswitches.

Tetrahydrofolic acid (5,6,7,8-tetrahydrofolic acid) is a vitamin B9 derivative that is a common one-carbon donor in organisms and is involved in growth and metabolism.