The role of the mitochondria in the generation of purines and thymidylate for DNA synthesis. Reactions 1, 3, 4, and 5 occur in both the mitochondria and the cytoplasm, with reaction 2 limited only to the mitochondria. Reaction 1 and 2 are catalyzed by SHMT2 and the glycine cleavage system, respectively. In the cytoplasm, reactions 3, 4, and 5 are carried out by the trifunctional enzyme (MTHFD1); in the mitochondria, 2 enzymes are required (MTHFD2 catalyzes reactions 3 and 4 and MTHFD1L catalyzes reaction 5). The enzyme 10-formyl tetrahydrofolate dehydrogenase (not shown) has been reported to be absent in most cancer cells (10). Serine hydroxymethytransferase (1); glycine oxidase complex (2); 5–10 methylene tetrahydrofolate dehydrogenase (3); methenyltetrahydrofolate cyclohydrolase (4); and formyltetrahydrofolate synthetase (5).