1. An isolated nucleic acid encoding osteoprotegerin (OPG) binding proteins selected from the group consisting of: a) the nucleic acids as shown in Fig. 1 (SEQ ID NO: 1) and Fig. 4 (SEQ ID NO: 3); b) nucleic acids which hybridize to the polypeptide coding regions of the nucleic acids shown in Fig. 1 (SEQ ID NO: 1) and Fig. 4 (SEQ ID NO: 3); and remain hybridized to the nucleic acids under high stringency conditions; and c) nucleic acids which are degenerate to the nucleic acids of (a) or (b). 2. The nucleic acid of claim 1 which is cDNA, genomic DNA, wholly or partially synthetic DNA, and RNA. 3. The nucleic acid of Claim 1 including one or more codons preferred for Escherichia coli expression. 4. The nucleic acid of Claim 1 having a detectable label attached thereto. 5. A nucleic acid of Claim 1 encoding a polypeptide comprising the amino acid sequence of residues 1-316 as shown in Figure 1 (SEQ ID NO:1). 6. A nucleic acid encoding a polypeptide comprising the amino acid sequence of residues 70-316 as shown in Figure 1 (SEQ ID NO:1). 7. A nucleic acid of Claim 1 encoding a polypeptide comprising amino acid sequence of residues 1-317 and residues 69-317 as shown in Figure 4 (SEQ ID NO:3). 8. A nucleic acid encoding a soluble OPG binding protein polypeptide comprising amino acid sequence of residues 69-317 as shown in Figure 4 (SEQ ID NO:3). 9. The nucleic acid encoding a soluble OPG binding protein comprising residues 69-317 as shown in Figure 4 (SEQ rD NO:3) and truncations thereof which stimulate forming osteoklasts or bone resorption. 10. An expression vector comprising the nucleic acid of Claims 1 and 9. 11. The expression vector of Claim 10 wherein the nucleic acid comprises the polypeptide-encoding region as shown in Figure 1 (SEQ ID NO:2_) and Figure 4 (SEQ ID NO:4). 12. A host cell which is a eucaryotic or procaryotic cell transformed or transfected with the expression vector of Claim 10. 13. The host cell of Claim 12 which is Escnencnia coli. 14. A process for the production of an osteoprotegerin binding protein comprising: growing under suitable nutrient conditions host cells of Claim 12, and isolating the polypeptide product of the expression of the nucleic acid. 15. A polypeptide produced by the process of Claim 14. 16. An isolated osteoprotegerin binding protein, being encoded by a nucleic acid or a fragment, analog, or derivative thereof which stimulates forming osteoklasts or bone resorption. 17. The protein of Claim 16 which has been covalently modified with a water-soluble polymer. 18. The protein of Claim 17 wherein the polymer is polyethylene glycol. 19. The protein of Claim 16 which is a soluble osteoprotegerin binding protein. 20. The protein of Claim 19 comprising the amino acid sequence from residues 70-316 inclusive as shown in Figure 1 (SEQ ID NO:2), or a fragment, analog, or derivative thereof. 21. The protein of Claim 19 comprising the amino acid sequence from residues 69-317 inclusive as shown in Figure 4 (SEQ ID NO:4) and truncations thereof. 22. The protein of Claim 19 comprising the amino acid sequence from residues 140-316 inclusive as shown in Figure 1 (SEQ ID NO:4), or a fragment, analog, or derivative thereof. 23. The protein of Claim 19 comprising the amino acid sequence from residues 145-316 inclusive as shown in Figure 1 (SEQ ID NO:4), or a fragment, analog, or derivative thereof. 24. An antibody or fragment thereof which specifically binds an osteoprotegerin binding protein of Claim 16. 25. A method for detecting the presence of an osteoprotegerin binding protein in a biological sample comprising: incubating the sample with the antibody of Claim 24 under conditions that allow binding of the antibody to the osteoprotegerin binding protein; and detecting the bound antibody. 26. A method for detecting the presence of osteoprotegerin in a biological sample comprising: incubating the sample with an osteoprotegerin binding protein of Claim 16 under conditions that allow binding of the protein to osteoprotegerin; and measuring the bound osteoprotegerin binding protein. 27. A method to assess the ability of a candidate compound to bind to an osteoprotegerin binding protein comprising: incubating the osteoprotegerin binding protein of Claim 16 with the candidate compound under conditions that allow binding; and detecting the bound compound. 28. The method of Claim 27 wherein the compound is an agonist or an antagonist of an osteoprotegerin binding protein. 29. A method of regulating expression of an osteoprotegerin binding protein in an animal comprising administering to the animal an effective amount of a nucleic acid of Claim 1 b). 30. A pharmaceutical composition comprising a therapeutically effective amount of an osteoprotegerin binding protein of Claim 16 in a pharmaceutically acceptable carrier, adjuvant, solubilizer, stabilizer and/or anti-oxidant. 31. The composition of Claim 30 wherein the osteoprotegerin binding protein is a human osteoprotegerin binding protein. 32. A method of preventing or treating bone disease in a mammal comprising administering a therapeutically effective amount of a modulator of an osteoprotegerin binding protein, wherein said modulator is the antibody of Claim 24, which inhibits forming osteoklasts or bone resorption. 33. A method to assess the ability of a test compound to increase or decrease binding of osteoprotegerin binding protein to osteoclast differentiation and activation receptor (ODAR) comprising: incubating osteoprotegerin binding protein, ODAR and optionally the test compound under conditions that allow binding of osteoprotegerin binding protein and said factor, and detecting the binding of osteoprotegerin binding protein to ODAR in the absence and presence of the test compound.