The methenyltetrahydrofolate cyclohydrolase activity of the trifunctional MTHFD1 enzyme catalyzes the reversible reaction of 10-formylTHF polyglutamate to form 5,10-methenylTHF polyglutamate and H2O. 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).