CD274 and nonpapillary renal cell carcinoma: Crucially, the iFRET assay was able to detect a PD-1/PD-L1 interaction in 10 of the 11 PD-L1 negative ccRCC patients.5 This demonstrates that ligand expression is a poor surrogate of receptor engagement and that the direct functional PD-1/PD-L1 interaction needs to be quantified.