Biologically active vitamin D metabolites mediate essential parts of their effect through binding to the vitamin D receptor (VDR) and other specific nuclear receptors (e.g., aryl hydrocarbon receptor [AhR], liver X receptor [LXR]). These so-called transcription factors are activated by the binding of vitamin D metabolites, enabling them to regulate several thousand target genes. The detection of these receptors identifies cell types and organs as targets of vitamin D action.