Herein, using bleomycin-induced lung fibrosis, we demonstrate that PDGFR-β+ cells are the major source of myofibroblasts (Fig. 1), and KLF4 in PDGFR-β+-derived cells is critical for the accumulation of excess myofibroblasts and ECM (Figs. 2–4). The gene discussed is PDGFRB; the disease is pulmonary fibrosis.