Phylogeny of vitamin D receptors (VDRs, NR1I), pregnane X receptors (PXRs,NR1I2), and constitutive androstane receptors (CARs, NR1I3). See Methods for details on the phylogenetic analysis. Four key evolutionary transitions in ligand sensitivity (i.e., ability to be activated by ligands) are proposed and indicated by the labels A, B, C, and D underneath four branches. Label A indicates loss of ligand sensitivity in the Xenopus frog PXRs relative to other PXRs. The Xenopus laevis and Xenopus tropicalis PXRs have narrow ligand sensitivity (essentially to benzoate compounds only) compared to other vertebrate PXRs. Label B indicates broadening of ligand specificity relative to the ancestral receptor. This is supported by the broader ligand specificity of vertebrate PXRs relative to the Ciona intestinalis VDR/PXR. Label C indicates acquisition of sensitivity to certain bile acids, particularly lithocholic acid and its derivatives, in mammalian VDRs. All non-mammalian VDRs studied so far are insensitive to bile salts. Label D indicates acquisition of sensitivity to vitamin D, a property of all vertebrate VDRs but not the Ciona VDR/PXR.