Schematic overview of enhancer and gene trapping. Normally a wild-type gene produces an unspliced RNA transcript that is processed by linking splice acceptor (SA) sites to splice donor (SD) sites. Transcription initiates at the promoter (Pro) and is regulated by enhancer sequences that can be located thousands of base pairs away from the promoter. (AAAAAAA) Poly-(A)-tail. (A) An enhancer-trapping element contains a minimal promoter and a translation start site. When the element integrates near an endogenous enhancer, that enhancer controls expression of the reporter gene. (B) In the basic gene-trapping element, a reporter gene sequence is flanked by splice acceptor (SA) and polyadenylation (pA) sequences. When the trapping element integrates within an intron, a fusion transcript is generated as a result of splicing of the endogenous splice donor with the vector-provided acceptor. As with enhancer trapping, the result is expression of the reporter gene that is faithfully controlled by the regulatory elements of the trapped gene locus. (C) The downstream sequences of the trapped gene can also be captured in a fusion transcript by providing a splice donor in the trapping element.