P{GR} and P{RG} together comprise the twin spot generator (TSG) system that can be used to directly and positively mark both daughter cells after mitotic division. P{GR} contains a cassette in which an Act5C promoter is fused upstream of sequence encoding the N-terminal part of EGFP and the C-terminal part of mRFP1, with the two partial coding sequences separated by an intron that contains an FRT site. P{RG} contains a cassette in which an Act5C promoter is fused upstream of the N-terminal part of mRFP1 and the C-terminal part of EGFP, with the two partial coding sequences separated by an intron that contains an FRT site. In the absence of FLP recombinase, neither P{GR} or P{RG} produces a fluorescent protein. In the presence of FLP recombinase, recombination between the FRT sites in a P{GR} element and a P{RG} element reconstitutes full-length coding sequences for EGFP and mRFP1 (as the fragments in P{GR} and P{RG} are complementary), resulting in recombinant cells being marked by fluorescence. The color depends on the stage at which recombination takes place and the subsequent segregation of the recombined chromosomes; red and green twin spots represent G2-X segregation, while doubly fluorescent yellow cells are generated by G1 or G0 recombination, or G2-Z segregation.