[Cytokines in bone diseases. Osteoprotegerin and juvenile Paget's disease].

Juvenile Paget's disease (JPD) is a rare autosomal-recessive hereditary disease and is typically diagnosed in infants or young children. JPD causes bone pain, bone expansion and deformity, and severity generally increases during adolescence. Recently, differentiation and maturation of osteoclasts, controlled by RANK, which is expressed in osteoclast precursors and its ligand, RANKL, which is expressed in osteoblasts or marrow stromal cells, have been clarified. Recent studies elucidated that JPD was caused by mutation of TNFRSF11B, which encodes osteoprotegerin, a soluble decoy receptor of RANKL. We summarize the outline and etiology of JPD.