FGFR4 and renal cell carcinoma: FGF19, along with FGFR4, is believed to be involved in the progression of hepatocellular carcinoma,37 and increased FGF21 levels have been investigated as a potential biomarker for renal cell carcinoma.38 These associations with cancer make FGF19 and FGF21 interesting potential biomarkers for FGFR inhibitors; we are not aware of any published data on FGF19 and FGF21 changes in patients treated with FGFR inhibitors.