NKX2-1 and pleuropulmonary blastoma: In the mouse, induction of epithelial FGF9 expression from E16.5 to E18.5 resulted in mesenchymal hyperplasia beneath a benign-appearing Nkx2.1 positive epithelium (Fig 6G and 6H) with histological features that were virtually identical to those observed in Type I or cystic PPB (Fig 6A and 6B).