FGF23 and chronic kidney disease: We cannot easily explain the induction of FGF23 expression in the kidneys of the rats with CKD by the high serum P and 1,25(OH)2D3 levels in this study because the serum P and 1,25(OH)2D3 levels in the hemi-nephrectomized rats fed the HP diet were high, but normal and low levels were observed in the partial nephrectomy rat model, and HK2 cells stimulated with high-P or 1,25(OH)2D3 did not express FGF23 mRNA.