The reactions of the oxidative branch of the pentose phosphate pathway occur in the cytosol and are important for generating NADPH through the oxidative reduction of NADP+. NADPH is an important source of the reducing energy required by many enzymes in central biosynthetic pathways. Although there are other biochemical pathways that generate NADPH in yeast, the pentose phosphate pathway is considered the major source of NADPH, and insufficient NADPH probably contributes to the fitness defects seen in mutants of the oxidative branch of the pentose phosphate pathway.