The GAL4::ER driver encodes a ligand-inducible transcriptional activator designed for use in a binary expression system. It is composed of the DNA-binding domain from the Saccharomyces cerevisiae GAL4 gene ( SGDID:S000006169 ) fused to the ligand-binding domain from the Homo sapiens estrogen receptor 1 gene (ESR1, HGNC:3467). The GAL4 DNA-binding domain of the GAL4::ER driver can bind specifically to the UAS regulatory sequence, but the presence of the ESR1 ligand-binding domain means that this binding, and subsequent transcriptional activation, can only occur in the presence of estradiol (PMID:3390864).