Another research project we are exploring regulation of transcription by bile acids in HepG2 cells. A recombinant cell line (human hepatocarcinoma HepG2) is used for assessing bile acids ability of activating the Farnesoid receptor (FXR) is co-transfected with two plasmids. The first bears the FXR. sequence to cause the cells to over-express the receptor; the second vector plasmid contains a portion of the I-BABP promoter and the firefly luciferase sequence as a reporter gene (I-BABP-Luc), so that the activity of FXR agonists or antagonists can be assessed by the luciferase expression. This system is utilized for the screening of natural and synthetic bile acids and could contribute to bile acid structure-activity relationship studies and facilitate a rational approach to the development of new drugs. It should be predictive of human pathophysiological conditions, being based on human cells, and it is rapid to carry out in comparison to animal-based tests, thus allowing the screening of many compounds.