The Golic FLP- FRT system demonstrates the principle for generating custom chromosomal deletions. (a) A collection of fly strains containing the two variants of the RS P element are generated and mapped to the genome by sequencing. In the case of RS5, the 3' and 5' regions of the white gene (w; shaded boxes) are separated by an arrangement of FRT sites (black arrows) such that the activity of FLP recombinase in cis generates a remnant element RS5r containing the 5' end of white followed by a single FRT site; the small fragment containing the 3' end of white is lost. The RS3 element is similar except that the action of FLP generates an RS3r remnant containing the 3' end of white preceded by an FRT site. Grey triangles represent the ends of the P element vector. (b) When remnants located up to 1 Mb apart on homologous chromosomes are combined together in a single fly, the activity of FLP recombinase on the two FRT sites in trans generates a reconstituted white gene, deleting the genomic material lying between the two insertions sites. The deletion is easily identified by the presence of red-eyed flies. The reciprocal product, a tandem duplication, does not have a white gene and is relatively unstable. The figure is adapted from Golic and Golic (1996) .