MMP9 and neoplasm: To examine the mechanism of bone resorption induced by tumour cells, we established a new coculture system of mouse calvaria and MDA-231 cells embedded in collagen gel, and found that MDA-231 cells markedly induced the bone-resorbing activity, the expression of RANKL in calvaria and the production of MMPs including MMP-2, MMP-9 and MMP-13 by calvaria (Figure 3 and Figure 4).