The P{PT1} element contains the following sequences from 3' to 5'; an artificial splice acceptor site, an FRT2 site, 'stop-start' sequence, a promoterless GAL4 driver coding sequence with an Hsp70 poly(A) signal, a second FRT2 site (in the same orientation as the first), the w^+mC marker, UAS regulatory sequences, a FRT1 site, the GFP coding sequence, a second FRT1 site (in the same orientation as the first). The sequence from the splice acceptor site through the GAL4 sequence forms a gene trap cassette; upon integration into the genome, the GAL4 coding sequence can be transcribed as a fusion mRNA with an upstream exon sequence, while the 'stop-start' sequence ensures that the open reading frame of GAL4 is maintained in any integration event. The UAS-GFP sequence acts as an internal marker for a successful gene trap event, since the GAL4 driver produced in this case will drive expression of the GFP fluorescent protein. The FRT1 and FRT2 sites are mutually exclusive, but are both targets of the native FLP recombinase. Thus, in the presence of FLP, both the sequence flanked by the two FRT1 sites, and the sequence flanked by the two FRT2 sites are expected to be excised, leaving the w^+mC marker and the UAS regulatory sequences. The resulting P{PT1Δ} element can act as a a mis-expression element, with the remaining UAS sequences driving overexpression of the downstream genomic sequence in the presence of a GAL4 driver.