[5][6][7] PXR is a nuclear receptor whose primary function is to sense the presence of foreign toxic substances and in response up regulate the expression of proteins involved in the detoxification and clearance of these substances from the body.

PXR is a transcriptional regulator of the cytochrome P450 gene CYP3A4, binding to the response element of the CYP3A4 promoter as a heterodimer with the 9-cis retinoic acid receptor RXR.

[7][9] PXR is activated by a large number of endogenous and exogenous chemicals[8] including steroids (e.g., progesterone, 17α-hydroxyprogesterone, 17α-hydroxypregnenolone, 5α-dihydroprogesterone, 5β-dihydroprogesterone, allopregnanolone, corticosterone, cyproterone acetate, spironolactone, dexamethasone, mifepristone), antibiotics (e.g., rifampicin, rifaximin), antimycotics, bile acids, hyperforin (a constituent of St. John's Wort), and other compounds such as meclizine, paclitaxel, cafestol,[10] and forskolin.

[8] One of the primary targets of PXR activation is the induction of CYP3A4, an important phase I oxidative enzyme that is responsible for the metabolism of many drugs.

[19][20] This article incorporates text from the United States National Library of Medicine, which is in the public domain.