CLDN4 and idiopathic pulmonary fibrosis: For example, the aforementioned AT2-AT1 intermediate cells are rarely detected in normal lungs [26, 63, 64], but increased number of cells expressing the markers of this intermediate population, such as KRT8, CLDN4 and SFN, were detected in patient lungs of idiopathic pulmonary fibrosis (IPF) [24–26], and recent studies indicate that these aberrant intermediate cells might activate the surroundings fibroblasts and contribute to the formation of fibrosis [23, 65].