COL4A3 and hypophosphatemia: To determine additional FGF23-responsive genes in the kidney of Col4a3−/− mice, we compared microarray analysis of kidneys isolated from 12 week-old WT and Col4a3−/− mice with the renal transcriptome in Hyp mice, which have hypophosphatemia and elevated FGF23 caused by inactivating mutations of Phex in osteoblasts [40], and FGF23 transgenic mice [31].