Fig. 1. The ligand sensor system. A schematic representation of the two transgenes that comprise the ligand sensor system is depicted. Upon heat treatment, the hsp70 promoter directs widespread expression of the GAL4 DNA-binding domain (DBD) fused to a nuclear receptor ligand-binding domain (LBD). This fusion protein is able to bind to a GAL4 UAS response element on a second transgene, activating reporter gene expression in cells that contain the necessary ligands and/or co-factors. Reporter genes that encode nuclear GFP or ß-galactosidase are used to monitor GAL4-LBD ligand sensor activity in a cell-autonomous manner.