TCF21 and cystic fibrosis: Currently, other proteins are extensively used to identify the CF population: platelet-derived growth factor receptor alpha (PDGFRα), collagen1α1 (Col1a1), transcription factor 21 (Tcf21), and CD90 (Thy1 gene), although the latter only comprises around 60–70% of the total CF population [8,10,11,12].