EPAS1 and chronic kidney disease: Quantification of renal fibrosis using gray-scale images of Sirius red staining captured under a polarized microscope revealed that renal fibrosis in wild-type CKD mice at 4 weeks was significantly ameliorated by HIF-2α activation at a later period but not by HIF-2α activation from the beginning of CKD induction (Fig. 2B and Supplementary Fig. 2).