When inserted into an intron of an expressed gene, the gene trap cassette is transcribed from the endogenous promoter of that gene in the form of a fusion transcript in which the exon(s) upstream of the insertion site is spliced in frame to the reporter/selectable marker gene.
When integrated into an intron of a transcribed gene, the gene trap cassette is expressed from the endogenous promoter of that gene as a fusion transcript in which the exon(s) preceding the insertion point is spliced in frame with the reporter/selectable marker gene.