Isolated nucleic acid (I) encoding an osteoprotegrin binding protein (II): (a) has a 2295 bp sequence (S1) or 2270 bp sequence (S2) (given in the specification) (b) hybridises under stringent conditions to (a), or (c) is a degenerate form of (a) or (b). Also new are: (1) (II), and their fragments, analogues and derivatives (2) nucleic acid (Ia) encoding a fragment or soluble form of (II) (3) expression vector containing (I) or (Ia) (4) host cells transfected or transformed with this vector (5) antibodies (Ab), or fragments, that bind specifically to (II), and (6) method for treating or preventing bone disease by administering a modulator of the osteoclast differentiation and activation receptor (ODAR).
