In addition, using Dpp4 knockout (Dpp4 KO) mice, we have revealed the functional roles of CD26/DPP4 in bleomycin-induced PH associated with interstitial lung disease, bleomycin-induced pulmonary fibrosis, and LPS-induced lung injury [23,24,28]. The gene discussed is DPP4; the disease is pulmonary fibrosis.