Paradoxically, in rheumatoid arthritis (RA), OPN produced by synovial fibroblasts in the synovial lining layer and at sites of cartilage invasion not only mediates attachment of these cells to cartilage, but also contributes to matrix degradation in RA by stimulating the secretion of collagenase 1 in articular chondrocytes (18). The gene discussed is SPP1; the disease is rheumatoid arthritis.