哺乳动物蛋白表达系统,具备蛋白折叠和翻译后修饰的功能，使其蛋白更接近天然蛋白，从而取得与天然蛋白相同的生物活性。因此使得哺乳动物细胞表达系统在重组蛋白药物，特别是治疗性重组单抗药物的研发和生产中有最为广泛的应用。

将含有pCspA1-ZsGreen、pCspA2-ZsGreen、pCspA3-ZsGreen的大肠杆菌（E.coli），在16℃表达；含有pT7-ZsGreen的大肠杆菌（E.coli）在37℃下表达至OD600＝0.6时，加入IPTG诱导表达，不加IPTG诱导的作为对照。

Nuclear receptors are ligand-activated transcription factors involved in regulating many physiologic and pathologic processes. Ligand binding to nuclear receptors leads to dissociation of co-repressors, recruitment of co-activators, and subsequent activation of gene expression. Because of their associations with many human diseases, nuclear receptors are therapeutic targets for pharmaceutical development. The pregnane X receptor (PXR, NR1I2) belongs to the nuclear receptor family. The activity of PXR can be controlled by the binding of small molecule agonists or antagonists. Because of its unique ligand binding pocket, PXR binds promiscuously to structurally diverse chemicals. PXR regulates the expression of proteins such as drug metabolizing enzyme cytochrome P450 3A4 (CYP3A4), efflux transporter P-glycoprotein, and other multidrug resistance proteins, which are involved in metabolism and elimination of potentially harmful chemicals. PXR has been detected in various tissues including kidney, colon, brain capillaries, small intestine, and predominantly in liver, and it can be activated by various ligands that bind to its ligand binding domain (LBD). PXR forms a heterodimeric complex with RXR to activate gene transcription. The recombinant pETDuet1-hPXR-LBD/mSRC-1 plasmid (Medicilon, Shanghai, China) was transformed into Escherichia coli BL21 DE3 cells for protein expression.