Which vitamin is required for transfer of 1-carbon unit?

Tetrahydrofolate can carry one-carbon fragments attached to N-5 (formyl, formimino, or methyl groups), N-10 (formyl) or bridging N-5-N-10 (methylene or methenyl groups). 5-Formyl-tetrahydrofolate is more stable than folate and is therefore used pharmaceutically (known as folinic acid), and the synthetic (racemic) compound (leucovorin).The major point of entry for one-carbon fragments into substituted folates is methylene-tetrahydrofolate.