TWIST1 and idiopathic pulmonary fibrosis: Immunohistochemistry studies on human IPF tissues localized vimentin-expressing epithelial cells in the area of fibroblast foci [30] and furthermore the presence of the EMT molecules smooth muscle actin/ACTA2 or TWIST [31], thereby suggesting EMT taking place, but leaving the origin of the collagen-secreting cell elusive.