Disclosed is a human osteoclast-derived cathepsin (Cathepsin O) polypeptide and DNA(RNA) encoding such cathepsin O polypeptides. Also provided is a procedure for producing such polypeptide by recombinant techniques. The present invention also discloses antibodies, antagonists and inhibitors of such polypeptide which may be used to prevent the action of such polypeptide and therefore may be used therapeutically to treat bone diseases such as osteoporosis and cancers, such as tumor metastases.