Thymidylate synthase is an enzyme that acts as a catalyst in the thiamine DNA conversion. Cells that are producing DNA need larger amounts of thymidylate synthase to fuel the DNA production. In the cell cycle more thymidylate synthase (20% more) is produced during S and G2 phase because this is the phase when the most rapid cell proliferation takes place. In some cancers, such as those of the gastrointestinal tract (such as gastric and rectal), thymidylate synthase is being studied as a possible tumor marker. Normal gastrointestinal tract cells are rapid proliferators, and tumors in these tissues can be very rapid in growth, elevating thymidylate synthase expression.