The GAL4::GS entry in FlyBase represents an artificial ligand-inducible transcriptional activator ('driver') that includes the DNA-binding domain from the Saccharomyces cerevisiae GAL4 gene ( SGDID:S000006169 ), a truncated ligand-binding domain from the Homo sapiens progesterone receptor (PGR, HGNC:8910) and the transcriptional activation domain from the Homo sapiens RELA gene (HGNC:9955). The GAL4 DNA-binding domain of the GAL4::GS driver can bind specifically to the UAS regulatory sequence, but the presence of the PGR ligand-binding domain means that this binding and subsequent transcriptional activation can only occur in the presence of the anti-progestin mifepristone (RU486) (PMID:9892637, FBrf0139796, FBrf0139797, also reviewed in PMID:9478886, FBrf0233764). The exact sequence of the driver may differ depending on the particular transgene or modified endogenous locus being used.