MMP13 and rheumatoid arthritis: Wnt signaling (Clevers, 2006), one of the most critical signaling pathway in rheumatoid arthritis (RA) pathogenesis, is a potent pathway to regulate the expression of matrix metalloproteinase-13 (MMP-13) (Wernicke et al., 2006), while MMP-13 is known to be one of key factors responsible for degradation of collagen type II in articular cartilage.