Derivatives of tetrahydrofolate (FH4) are involved in a variety of biosynthetic reactions. Inhibition of the enzyme dihydrofolate reductase (DHFR) depletes the available tetrahydrofolate and blocks the formation of thymidylate, purines, the amino acids methionine and glycine, and several other cell constituents. Lack of thymidylate disrupts DNA synthesis and cell growth ceases. Many folate analogues are competitive inhibitors of DHFR, and while some classes show no selectivity, suitably substituted diaminopyrimidines are several thousands times more active against bacterial than mammalian DHFR and are thus effective and safe antibiotics. The ease of isolation, stability and low molecular weight of DHFR has facilitated its study at a molecular level. Considerable knowledge has been accumulated about the interactions of inhibitors with the protein chain in and around the active site, but despite the availability of many sequences and several crystal structures the exact details of how subtle variations of inhibitor structure result in such large differences in affinity for the enzyme are not yet clear.