FGF9 and atrial fibrillation: Other significant molecules implicated in mediating ECM synthesis in AF include growth factors such as platelet-derived growth factor beta (PDGFB), vascular endothelial growth factor C (VEGFC) and fibroblast growth factor 9 (FGF9), and multiple signaling proteins such as the neurogenic locus notch homolog protein (NOTCH) and WNT [45].