Phosphoribosylpyrophosphate synthetase (PRS) catalyzes the phosphoribosylation of ribose 5-phosphate to 5-phosphoribosyl-l-pyrophosphate (PRPP), which is necessary for the de novo and salvage pathways of purine, pyrimidine and pyridine biosynthesis (1). In the rat, two nearly identical cDNA transcripts (PRS I and PRS II) have been cloned (2). Additionally, a human cDNA homologous to the rat PRS II cDNA has been cloned (3). We report the isolation of a human PRS cDNA homologous to rat PRS I and confirm the nucleotide sequence of an independently isolated human PRS II cDNA. Published rat cDNA sequences were used to design flanking oligonucleotide primers, and a partial length human PRS cDNA probe was generated from human lymphoblast mRNA by the polymerase chain reaction (4). Human PRS I was isolated from lymphoblast cDNA libraries cloned into lambda gtlO. Human PRS II was isolated from human kidney cell cDNA libraries cloned into lambda gtlO. Both human PRS I and PRS II cDNA were sequenced by the Sanger method (5).