Fig. 1. Schematic representation of the GAL4-LBD system. The upper diagram depicts a fusion gene consisting of the coding region for the yeast GAL4 DNA-binding domain (GAL4 DBD) fused in-frame with the ligand binding domain (LBD) of EcR or USP. This construct is introduced into the Drosophila genome under the control of the hsp70 promoter. Heat-induced GAL4-LBD protein can bind to GAL4 upstream activating sequences (UAS) that are present in a second transgenic construct. In the absence of ligand, the GAL4-LBD protein does not activate lacZ reporter gene transcription; however, in the presence of its ligand (black circle) it can switch into an active conformation and induce lacZ expression.