P{Mi.GT-CaSSA.3x1-y-3x2} contains a gene trap cassette composed of three copies of the 'gRNA#1' guide RNA (flanked by the hammerhead and HDV ribozymes) and three copies of the 'gRNA#2' guide RNA (flanked by the hammerhead and HDV ribozymes) separated by the y^+mDint2 marker. The 'gRNA#1' and 'gRNA#2' components are in opposite orientations, meaning that the gene trap cassette can potentially report sense and antisense transcription when inserted in the genome and combined with an appropriate CaSSA reporter containing a switch cassette with 'gRNA#1' and/or 'gRNA#2' target sites. The gene trap cassette (including the y^+mDint2 marker) are flanked by inverted attP sites, which allows for the replacement of this entire sequence with DNA from a compatible donor plasmid (where the sequence to be inserted is flanked by inverted attB sites) through recombination-mediated cassette exchange (RMCE) driven by the phiC31:int integrase. Outside the attP sites are Dhyd\Minos element terminal inverted repeats, which are themselves flanked by P-element terminal inverted repeats, meaning that the P{Mi.GT-CaSSA.3x1-y-3x2} transgene can be mobilized using either P\T or Dhyd\Minos\T transposases.