nSyb promoter sequence is fused upstream of an FRT cassette that contains the His2Av coding sequence tagged with mCherry and followed by an SV40 termination sequence. Sequence encoding the QF2w driver is located downstream of the FRT cassette. In the absence of FLP, only the mCherry-tagged His2Av marker is expressed under the control of the nSyb regulatory sequences. If the FRT cassette is excised (removing mCherry-tagged His2Av), the QF2w driver is expressed.