In the genetically induced murine Col4a3 CKD knock‐out model, increases in circulating iFGF23 levels have been shown to precede an increase in Fgf23 mRNA levels in bone,49 suggesting the possibility that FGF23 production in non‐osseous tissues may contribute to the CKD‐induced increase in circulating iFGF23 levels. This evidence concerns the gene COL4A3 and chronic kidney disease.