The FLP/FRT system exploits the yeast flippase (FLP) recombinase to generate mitotic recombination at FLP-recombinase target sequences (FRTs). Combining the FLP/FRT system for genetic recombination with the GAL4/UAS system for transgene expression [3] allows the generation of mutant clones in a given cell type that express detectable markers, thus providing spatial, temporal, and genetic information about individual cells.