The pentose-phosphate pathway of Escherichia coli K-12, in addition to its role as a route for the breakdown of sugars such as glucose or pentoses, provides the cell with intermediates for the anabolism of amino acids, vitamins, nucleotides, and cell wall constituents. Through its oxidative branch, it is a major source of NADPH. The expression of the gene for NADP-dependent 6-phosphogluconate dehydrogenase (gnd) is regulated by the growth rate in E. coli. The recently identified gene for ribulose-5-phosphate 3-epimerase (rpe) is part of a large operon that comprises among other genes for the biosynthesis of aromatic amino acids. In recent years, genes for all enzymes of the pathway have been cloned and sequenced. Isoenzymes have been found for transketolase (genes tktA and tktB), ribose-5-phosphate isomerase (rpiA and rpiB) and transaldolase (talA and talB).