Mammalian mitochondria are able to produce formate from one-carbon (1C) donors such as serine, glycine and sarcosine. In mitochondria, the 1C units are oxidised to formate and transported to the cytosol, where the formate is reattached to tetrahydrofolate (THF) for use in de novo purine biosynthesis. The mitochondrial bifunctional methylenetetrahydrofolate dehydrogenase/cyclohydrolases D2 and D2L (MTHFD2, MTHFD2L) mediate reversible reactions to convert 5,10-methylene-THF, via 5,10-methenyl-THF, to 10-formyl-THF. These enzymes have filled in missing gaps which existed in the transformation of 5,10-methylene-THF to formate in human mitochondria (Yang & MacKenzie 1993, Bolusani et al. 2011, Shin et al. 2014).