Since vitamin D deficiency is a hallmark of CKD, we quantified the expression of the vitamin D receptor (Vdr), the ligand for the active vitamin D metabolite 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), as well as the two genes involved in the metabolism of vitamin D in the kidney, Cyp27b1, which catalyzes the conversion 25(OH)D3 to 1,25(OH)2 D3, and Cyp24a1, responsible for the inactivation of 1,25(OH)2D3 into 24,25(OH)2D3 (Figure 5B). Here, CYP27B1 is linked to chronic kidney disease.