All C1-tetrahydrofolate (C1-THF) synthases characterised to date are trifunctional, containing the activities of 5,10-methylene-THF dehydrogenase, 5,10-methenyl-THF cyclohydrolase, and 10-formyl-THF synthetase. Mitochondrial monofunctional C1-tetrahydrofolate synthase (MTHFD1L) only possesses 10-formyl-THF synthetase activity and it reversibly ligates formate (HCOOH) to tetrahydrofolate (THF), forming 10-formyltetrahydrofolate (10-formyl-THF) (Prasannan et al. 2003, Walkup & Appling 2005). MTHFD1L is functional as a homodimer and could be a missing reaction in one-carbon folate metabolism linking the metabolism of formate from the cytosol to mitochondria (Pike et al. 2010). Under most conditions, the majority of one-carbon units for cytoplasmic processes are derived from mitochondrial formate.