Insertional mutagenesis strategies used in vertebrates. In each case, a schematized endogenous locus is represented by exons (E) and an endogenous regulatory element. A nonintegrated vector is also shown above, with integrated vector below. Transcriptional start sites are shown as an arrow above each diagram. (a) Integration of DNA into a coding exon can mutate the locus, resulting in a truncated gene product. (b) Retroviral insertional mutagenesis alters the tagged locus using multiple methods, including the loss of the encoding transcript. (c) 5' gene trapping in mouse embryonic stem cells. Shown is one approach whereby the resulting fusion transcript encodes a truncated gene product fused to the selectable marker protein. (d) Insertional mutagenesis in zebrafish using transposons. Based on 3' exon or poly(A) trapping methods, this approach uses two components: a transcriptional termination cassette to truncate the integrated locus and a separate 3' exon trap gene finding cassette. See text for details. pA, polyadenlyation signal; SA, splice acceptor; SD, splice donor.