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 placed within an intron of an expressed gene, the gene trap cassette is transcribed from its endogenous promoter and produces a fusion transcript that includes the exons preceding the insertion site spliced in frame with the reporter/selectable marker gene.