Proposed phylogeny of the NR1I subfamily. The phylogenetic tree is based on known phylogenetic relationships between the species combined with functional data from this study and others. Features included are activation by pregnane and androstane steroids, C27 bile alcohol sulfates (like cyprinol and scymnol sulfate), C24 bile acids (such as cholic and lithocholic acid), benzoates, and calcitriol; ability to increase (upregulate) the expression of the CYP3A enzymes; and high constitutive (baseline) activity. It is possible that activation of PXRs by benzoates is an ancestral property as all PXRs can be activated by at least some benzoate compounds , although functional roles of these compounds have so far only been demonstrated in frogs [41, 42]. The study of ligand effects on CARs is complicated by the high constitutive activity of these receptors; many ligands act as inverse agonists of CARs. The possible developmental role of zebrafish PXR is highlighted by its strong expression in early life stages of zebrafish .