VDR and chronic kidney disease: Furthermore to the regulation of calcium metabolism, calcitriol is involved in various biological reactions such as differentiation induction, anti-proliferative effect, immunomodulatory effect, and regulation of cytokine and parathyroid hormone secretion, while, all of these disorders obviously increase in chronic renal failure because the kidney is essential to maintain serum levels of 1, 25-dihydroxyvitamin D3, the most potent endogenous endocrine activator of the VDR, and also of 25-hydroxyvitamin D, for local rather than systemic VDR activation.