Inactivating mutations of the neutral endopeptidase, PEX, have been identified as the cause of X-linked hypophosphatemia (XLH). Though the function of PEX is unknown, current information suggests that impaired renal phosphate conservation in XLH is due to the failure of PEX to either degrade an undefined phosphaturic factor or activate a novel phosphate-conserving hormone. The physiologically relevant target tissue for the XLH mutation has not been identified. An apparent intrinsic defect of osteoblast function in XLH implicates bone as a possible site of PEX expression. In the current investigation, we employed a polymerase chain reaction (PCR) strategy to amplify a PEX cDNA from a human bone cell cDNA library. We found that the human PEX cDNA encodes a 749 amino acid protein belonging to the type II integral membrane zinc-dependent endopeptidase family. The predicted PEX amino acid sequence shares 96.0% identify to the recently cloned mouse Pex cDNA and has 27-38% identity to other members of the metalloendopeptidase family. Using reverse transcriptase (RT)-PCR with PEX-specific primers, we detected PEX transcripts in both human osteosarcoma-derived MG-63 osteoblasts and in differentiated mouse MC3T3-E1 clonal osteoblasts but not in immature MC3T3-E1 preosteoblasts. The association of impaired mineralization of bone in XLH and the apparent developmental stage-specific expression of PEX in osteoblasts suggest that bone is a physiologically relevant site of PEX expression and that PEX may play an active role in osteoblast-mediated mineralization.