TCF21 and cystic fibrosis: Currently, other proteins are extensively used to identify the CF population: platelet-derived growth factor receptor α (PDGFRα), collagen1a1 (Col1a1) and collagen1a2 (Col1a2), transcription factor 21 (Tcf21), periostin (Postn) and CD90, although in this latter only comprises around 60–70% of the total CF population [6,16,17,18].