The main metabolic challenge for methylotrophs is the assimilation of single carbon units into biomass. Through de novo synthesis, methylotrophs must form carbon-carbon bonds between 1-Carbon (<chem>C1</chem>) molecules. This is an energy intensive process, which facultative methylotrophs avoid by using a range of larger organic compounds. However, obligate methylotrophs must assimilate <chem>C1</chem> molecules. There are four distinct assimilation pathways with the common theme of generating one <chem>C3</chem> molecule. Bacteria use three of these pathways while Fungi use one. All four pathways incorporate 3 <chem>C1</chem> molecules into multi-carbon intermediates, then cleave one intermediate into a new <chem>C3</chem> molecule. The remaining intermediates are rearranged to regenerate the original multi-carbon intermediates.