Abstract: A previously undiscovered transmembrane protein tyrosine phosphatase expressed only in bone and testis, osteotesticular protein tyrosine phosphatase (OST-PTP), has been identified, cloned, sequenced, expressed and subjected to enzymatic analysis. Also, a truncated osteoblast specific form (OST) containing the OST-PTP receptor but lacking the catalytic domain was identified and characterized. Based on the unique expression pattern of OST-PTP and OST during osteogenesis, assays can be used to screen for abnormal bone growth patterns and metabolic bone diseases. Moreover, this novel protein provides a specific target for use in conventional and gene therapies directed at treatment of osteoporosis, osteopetrosis and other bone metabolic disorders.