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Cytosolic phophoribosyl pyrophosphate synthetase catalyzes the reaction of D-ribose 5-phosphate and ATP to form 5-phospho-alpha-D-ribose and AMP. Three isoforms of the enzyme have been described. The first has been purified and characterized biochemically (Fox and Kelley 1971). The others are known only as inferred protein products of cloned genes; their catalytic properties have not been determined.