FGFR2 and nonpapillary renal cell carcinoma: reported a tissue-specific isoform switch of fibroblast growth factor receptor 2 (FGFR2) from FGFR2 IIIb (epithelial isoform) to FGFR2 IIIc (mesenchymal isoform) in nearly 90% of ccRCC patients.