Description_x000D_ Biochem/physiol Actions_x000D_ Farnesoid-X-activated receptor (FXR) was originally identified and cloned in rat as an orphan nuclear hormone receptor based on hybridization with a degenerate oligonucleotide designed from the highly conserved nuclear hormone receptor DNA binding domain. FXR functions as a heterodimer with RXR and binds to sequence elements in the promoters of target genes. The FXR/RXR heterodimer binds with highest affinity to inverted repeats separated by 1 bp (IR-1) and with low affinity to direct repeats separated by 4 and 5 bp (DR-4 and DR-5). As is the case for other nuclear hormone receptors, FXR regulates target gene activity in response to ligand. While initial studies suggested that farnesol and retinoid metabolites were likely ligands for FXR, current data support the notion that FXR is a bile acid sensor that plays an integral role in bile acid synthesis and transport. In the small intestine, FXR regulates bile acid uptake through the upregulation of the ileal bile acid binding protein gene via binding to an upstream response element. The FXR/RXR heterodimer can be activated by the bile salt chenodeoxycholic acid (CDCA) and FXR is required for the bile salt-dependent transcriptional control of the human ABCB11 gene (the bile salt export pump). In addition, FXR has been shown to inhibit the cholesterol 7-hydrolase gene (CYP7A1) transcription._x000D_ Physical form_x000D_ Clear and colorless frozen liquid solution_x000D_ Preparation Note_x000D_ Use a manual defrost freezer and avoid repeated freeze-thaw cycles. While working, please keep sample on ice.