Fig. (1) Reporter gene assay formats. A) A response element upstream of the reporter gene is induced by full length endogenous receptor or a receptor that is exogenously introduced. Upon ligand binding and activation by the ligand the nuclear receptor binds to the response element to activate transcription. B) The GAL4 hybrid format utilizes two plasmids. One contains a number of GAL4 UAS repeats upstream of the reporter gene. The second plasmid contains the LBD of the receptor of interest fused to the GAL4 DBD. Upon ligand binding the GAL4 DBD-NR-LBD fusion binds to the GAL4 UAS to activate transcription.