The vitamin D receptor (VDR). Within its target tissues, calcitriol binds to the VDR. The VDR is a 424 amino acid polypeptide. Its genomic information is encoded on the

12q12-14 chromosome, near the gene for the 1-a-hydroxylase enzyme. The VDR is found in the intestinal epithelium, parathyroid cells, kidney cells, osteoblasts, and thyroid cells. VDR also can be detected in keratinocytes, monocyte precursor cells, muscle cells, and numerous other tissues. The allele variations for the vitamin D receptor. Two allele variations exist for the vitamin D receptor (VDR): the b allele and the B allele. In general, normal persons with the b allele seem to have a higher bone mineral density [9]. Among patients on dialysis, those with the b allele may have higher levels of circulating parathyroid hormone (PTH) [7,9,10,11]. COOH—carboxy terminal; NH2—amino terminal. (From Root [7]; with permission.)

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