SOX9 and arthritic joint disease: In addition, a reduction in wif-1 expression was also shown to promote cartilage damage in experimental arthritis [33] suggesting that the reduction of wif-1, BMP-2 and BMP-7 expression together with reduced expression of Sox9, a master regulator of chondrocyte differentiation, in the cartilage in AS joints may be considered as signs of cartilage degeneration.