The P{CoinFLP-LexA::GAD.GAL4} system is designed to generate mosaic tissues composed of clones which express either the GAL4 driver or the lexA::GAD driver. The P{CoinFLP-LexA::GAD.GAL4} construct has the following structure: Act5C regulatory sequences - FRT site - FRT3 site - transcriptional STOP sequence - FRT site - lexA::GAD driver sequence - 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. In the absence of FLP recombinase neither the lexA::GAD driver nor the GAL4 driver is expressed, due to the presence of the STOP sequence. If excision of the FRT-flanked sequence occurs, the STOP sequence is removed, allowing expression of lexA::GAD (but not GAL4). If excision of the FRT3-flanked sequence occurs, the STOP sequence and lexA::GAD driver sequence are removed, allowing expression of GAL4 instead. GAL4-expressing clones are less frequent than lexA::GAD expressing clones after FLP-mediated recombination in eye-antennal discs. In polyploid tissues, cells co-expressing both GAL4 and lexA::GAD are often observed (presumably due to the presence of multiple copies of the P{CoinFLP-LexA::GAD.GAL4} construct in the same cell). The P{CoinFLP-LexA::GAD.GAL4} system can be combined with the GRASP system to visualise clonal boundaries.