The enzyme 25-hydroxyvitamin D 1-hydroxylase (CYP27B1) is the rate limiting enzyme in the two-step activation process of Vitamin D to its active form 1,25-dihydroxyvitamin D (1,25D) and is located in the mitochondrial fraction of the proximal tubular cells of the kidney. More recently CYP27B1 activity and expression have also been identified in a number of non-renal cells, which is suggestive of new, previously unidentified roles for Vitamin D in the human body. Although the regulation of CYP27B1 activity and expression has been a major focus of interest over the past decades, the exact molecular mechanism behind the regulation of CYP27B1 activity and expression and the role of the CYP27B1 promoter, herein, are still poorly understood. In this study, we created a transgenic mouse model that expresses the luciferase reporter gene under the control of the full-length, 1.5kb, human CYP27B1 promoter. This animal model allows us to study in vivo the tissue-specific, CYP27B1 promoter-controlled, regulation of the expression of the CYP27B1 gene.