BMP2 and rheumatoid arthritis: The finding that SF levels of OP-1 were higher in RA patients than in OA patients or asymptomatic donors is also consistent with recent reports that IL-1, which is present at higher concentrations in RA joints than in OA joints, is an effective modulator and/or stimulator of BMP-2 and OP-1 mRNA expression by normal and OA human articular chondrocytes [23,32,33].