EPAS1 and nonpapillary renal cell carcinoma: Furthermore, recently, HIF2α inhibitor-resistant ccRCC patient-derived xenograft tumors were observed to have significantly greater HIF1A mRNA expression than HIF2α inhibitor-sensitive ccRCC tumors, raising the possibility that HIF1α may be particularly oncogenic in ccRCC tumors with no response despite HIF2α inhibition43.