PSME4 and idiopathic pulmonary fibrosis: Immunohistochemistry analysis identified increased expression of PA200 in well-known IPF driver cells, i.e. α smooth muscle actin (αSMA)-positive myofibroblasts3 and keratin 5 (KRT5)-positive hyperplastic bronchial basal cells19 (Fig. 1B and Supplementary Fig. S2).