Possible consequences of transposon integration in or close to a transcription unit. (a) A hypothetical host genomic transcription unit with a promoter (red arrow) driving expression of Gene A. (b) Insertion of a transposon into the coding region results in a truncated gene product. This example shows a DNA transposon, but retroelement insertion can have similar consequences. The black arrows represent terminal inverted repeats flanking a transposase coding region (yellow box). (c) Transposon insertion into the 5' transcriptional regulatory region of the gene might introduce a binding site for a transcription factor (blue sphere), resulting in ectopic and/or overexpression of Gene A. (d) Transposition into multiple genes brings Genes A, B and C into a regulatory network under the control of a master transcriptional regulator. (e) The transposase coding region gets fused to a transcriptional regulatory domain, but can still bind to the inverted repeats of transposons dispersed in the genome. The transposase fusion protein might thereby become a master regulator of genes that have a transposon insertion.