PBac{IT.GAL4.w-} represents a transgenic construct generated in vivo by excising the w^+mC-marked loxP cassette from PBac{IT.GAL4}. It thus contains an enhancer trap cassette that consists of a minimal P\T promoter fused upstream of sequence encoding a GAL4 driver followed by the Hsp70 3' UTR, with the cassette flanked on the 5' side by a single attP site and on the 3' side by a single loxP site. PBac{IT.GAL4.w-} is a target element for the InSITE system; the GAL4 enhancer trap cassette can be swapped with another cassette from any compatible InSITE donor construct (this process involves using phiC31:int integrase to insert the new donor cassette into the attP site, and then the swap can be completed by using P1cre recombinase to remove the original target GAL4 coding region).