The P{UAS(loxP.lexA)QF} mosaic expression system is designed to generate mosaic tissues composed of clones which express either the lexA::VP16 driver or the QF driver. The P{UAS(loxP.lexA)QF} construct has the following structure: UAS regulatory sequences - loxP site - lexA::VP16 driver sequence (with polyadenylation signal) - loxP site - QF driver sequence (with polyadenylation signal). In the absence of P1cre recombinase, only the lexA::VP16 driver is expressed (represented in FlyBase by the Ecol\lexA^{VP16.UAS.loxP} allele). If excision of the loxP-flanked sequence occurs (generating the P{UAS(-loxP)QF} construct), the lexA::VP16 driver sequence is removed, allowing expression of the QF driver instead (represented in FlyBase by the Ncra\QF^UAS.-loxP allele).

UAS regulatory sequences drive expression of the lexA::VP16 driver (the driver coding sequence is also flanked by loxP sites).