The P{PromoterSwitch} system is designed to target transgene expression to a small subset of cells expressing a cell-type-specific GAL4 driver. The P{PromoterSwitch} construct contains 3 sequence cassettes: a PromoterSwitch cassette, Avic\GFP^UAS.cUa and Scer\FLP1^UAS.cUa. The PromoterSwitch cassette has the following structure: Act5C regulatory sequences - FRT site - FRT3 site - GAL80ts - FRT site - GAL80 - FRT3 site - GAL4 driver sequence. The FRT and FRT3 sites are mutually incompatible and are arranged such that a single FLP-mediated recombination event, excising either the FRT-flanked sequence or the FRT3-flanked sequence can occur in a stochastic manner, with no further recombination possible. If expression of FLP recombinase (encoded by the Scer\FLP1^UAS.cUa cassette) is induced using a cell-specific GAL4 driver carried in a different transgene, most excision events result in ubiquitous Act5C-driven expression of GAL80 (excision using the FRT sites), shutting down expression using the UAS-GAL4 system. However, in a small number of cells, excision using the FRT3 sites occurs, removing both GAL80ts and GAL80 and resulting in ubiquitous Act5C-driven expression of GAL4. Since this excision event only occurs in cells in which Scer\FLP1^UAS.cUa expression was induced by the cell-specific GAL4 driver, cell-type specificity of the genetic manipulations is preserved.