The formate-tetrahydrofolate ligase activity of the trifunctional MTHFD1 enzyme catalyzes the reaction of THF polyglutamate, formate, and ATP to form 10-formylTHF polyglutamate, ADP, and orthophosphate. MTHFD1 is cytosolic and occurs as a dimer. The human enzyme has been identified and partially characterized biochemically (Hum et al. 1988); additional reaction details can be inferred from the properties of the well-studied homologous rabbit enzyme (Villar et al. 1985).