PXR activation and steroid pathways. Steroid pathways typical of vertebrates are indicated. (A) Human PXR is activated by a large number of steroid hormones, although typically at micromolar concentrations. The coloring indicates at which concentrations the various steroids activate human PXR (see key in bottom right of panel). (B) Zebrafish PXR is activated by a smaller number of steroid hormones than human PXR, although there is much overlap between the selectivity of the two PXRs. Zebrafish PXR tends to be more sensitive to steroid hormone activation, at least for the functional assay used in this study. The coloring indicates at which concentrations the various steroids activate zebrafish PXR using the same key as in (A). Abbreviations: dehydroepiandrosterone, DHEA; DHEA sulfate; DHEA SO4; dihydrotesterone, DHT.