The strains carrying a construct including upstream activating sequence (UAS, β-gal-coding region downstream from the GAL4-binding sites). A responder gene downstream of UAS is expressed by crossing with a strain expressing GAL4, driven by specific promoters or by enhancer traps.
Some balancer chromosomes also have a UAS construct but they are not in this subset. Strains carrying a misexpression vector that includes UAS (GS lines and LA lines) are not in this subset.