Since it is well established that the activation of quiescent fibroblasts leads to the acquisition of a more differentiated phenotype and to an increased secretion of extracellular matrix components and proteases (for a recent review, see Räsänen and Vaheri 2010), we quantified by ELISA test, in cellular homogenates of CHO-AFs and control fibroblasts, the expression levels of the metalloproteases MMP-2 and MMP-9, which are known to be modulated in cholesteatoma tissues (Schönermark et al. 1996; Shibosawa et al. 2000). This evidence concerns the gene MMP9 and cholesteatoma.