There may be differences in the DRs of PTH and bone turnover markers in people with pathological conditions such as osteoporosis [28], [29] and diabetes [20] but this has so far not been shown for 1,25(OH)2D. Plasma concentrations of 1,25(OH)2D reflect the renal rather than the extra-renal production of 1,25(OH)2D. Other cell types which express the CYP27B1 and CYP24A1 may produce and catabolize 1,25(OH)2D intracellularly and thus may have an independent DR. This evidence concerns the gene CYP27B1 and osteoporosis.