In IPF, the stimulatory effect of the local composition of the ECM on fibroblast proliferation can either result from the mitogenic effect of collagen type I as shown in nondiseased human lung fibroblasts [11] or from the direct proproliferative effect of TGF-β1 released from alveolar epithelial cells [12]. The gene discussed is TGFB1; the disease is idiopathic pulmonary fibrosis.