The GAL4::LBD entry in FlyBase represents a ligand-inducible transcriptional activator that is composed of the DNA-binding domain from the Saccharomyces cerevisiae GAL4 gene ( SGDID:S000006169 ) fused to the ligand-binding domain from a Drosophila nuclear receptor gene. The GAL4 DNA-binding domain can bind specifically to the UAS regulatory sequence, but in the case of GAL4::LBD, the presence of the ligand-binding domain means that this binding, and subsequent transcriptional activation, can only occur in the presence of the ligand. GAL4::LBD thus acts as a 'sensor' that can be used to detect the spatial and temporal expression of the ligand in vivo. It can also be used as a driver to control the spatial and temporal expression of a gene of interest: a transgene or modified endogenous locus carrying the target gene of interest downstream of UAS sequences is combined with a second transgene or modified endogenous locus encoding GAL4::LBD. In the latter case, activation of the driver can either utilize the normal expression of the relevant ligand in vivo, or the ligand or an analog can be exogenously supplied to ectopically activate the driver (FBrf0193912).