Major types of expression and mutagenic cassettes delivered by transposon vectors for versatile applications. (A) A gene-trap transposon contains a splice acceptor (SA) sequence followed by a promoterless reporter gene such as lacZ and a poly-A (pA) signal. The reporter gene is expressed only when its transcription is initiated from the promoter of a disrupted, actively transcribed endogenous transcription unit. Therefore, the expression pattern of the transposon reporter reflects that of the endogenous gene harboring the transposon insertion. (B) Oncogene-trap transposons include a strong viral promoter/enhancer long terminal repeat (LTR) sequence that can drive transcription toward the outside of the transposon and a splice donor (SD) sequence, so that forced transcription and splicing to downstream exons will result in overexpression of a product of a gene into which the transposon has inserted. Classic oncogene trap vectors also contain SA and pA sites to induce gene truncations that have dominant phenotypes. (C) Knockdown expression cassette including a polymerase (Pol) II promoter that drives expression of a marker gene and a Pol III promoter that drives expression of a short hairpin (sh)RNA. (D) A typical therapeutic expression cassette contains a ubiquitous or tissue-specific enhancer/Pol II promoter that drives expression of a therapeutic gene. To enhance the safety of such a vector, the expression cassette might be flanked by insulator elements that will block transactivation of endogenous promoters by the transposon insertion, and simultaneously protect the expression of the therapeutic gene from position effects.