The nuclear receptors constitutive androstane receptor (CAR), pregnane X receptor (PXR) and vitamin D receptor (VDR) control a large array of genes that code for important proteins in humans including metabolic enzymes and transporters. 3D structures for the ligand-binding domain (LBD) of these receptors are abundantly available, providing valuable insights into the ligand-binding specificity as well as the activation mechanisms. The ligand-binding site of PXR is large and flexible, whereas those of CAR and VDR are compact and rigid, respectively. In general, the ligand profiles of the receptors are in agreement with the LBD structures. The crystal structures have greatly helped us to understand the promiscuity and/or specificity of CAR, PXR and VDR.
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