CYP27B1 and vitamin D deficiency: 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).