In the second step, CF epithelial cells were more proliferating than non-CF cells and this phenomenon was paralleled by an increase in IL-8, MMP-7, MMP-9 and TIMP-1, implicating these mediators in hyper-proliferation and suggesting that heightened proliferation is involved in the delay observed in CF epithelial differentiation. Here, MMP9 is linked to cystic fibrosis.