To analyse the molecular basis of this transcriptional specificity, we developed a CRISPR-Cas9-based knock-in strategy for the integration of an mCherry fluorescence reporter into the endogenous EPAS1 locus of ccRCC cells using homology-directed repair (Fig. 1b), an approach that has previously been successfully applied in other biological contexts14,15. The gene discussed is EPAS1; the disease is nonpapillary renal cell carcinoma.