The P{GTD-b} 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(GTD-b) hemidriver coding sequence and an Hsp70 poly(A) signal. Upon integration into the genome, the GAL4(GTD-b) 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(GTD-b) coding sequence to ensure that the open reading frame of GAL4(GTD-b) 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{GTD-b} 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.