The P{GT1} element contains two trapping cassettes. At the 3' end of the element there is a mutagenic gene trap cassette consisting of an artificial splice acceptor site, a promoterless GAL4 driver coding sequence and an Hsp70 poly(A) signal. Upon integration into the genome, the GAL4 coding sequence can be transcribed as a fusion mRNA with an upstream exon sequence. A 'stop-start' sequence is placed between the splice acceptor site and the GAL4 coding sequence to ensure that the open reading frame of GAL4 is maintained in any integration event. At the 5' end of the element, a polyA trap cassette is present that consists of the w^+mGT minigene (which has a promoter but lacks a poly(A) signal sequence) followed by an artificial splice donor site. The w^+mGT mRNA is only polyadenylated upon insertion of the P{GT1} element into the intron of a host gene, upstream of an exon containing poly(A) signal sequence, and thus w expression can be used as a marker for a successful integration into an intron in the correction orientation (since in the absence of polyadenylation the mRNA is likely to be rapidly degraded). A Tn\neoR^Hsp70Bb.PS marker is present between the two trapping cassettes.

A diagram of the P{GT1} element is available at the [GDP Transposons page](http://flypush.imgen.bcm.tmc.edu/pscreen/transposons.html)