Diagrammatic representation of the TMDH process. a) Illustration of the mariner transposon b) A saturated genome-wide transposon library is produced. Cells with a transposon disrupting an essential gene will not survive. Genomic DNA from the library is restriction digested, and Cy5-labelled RNA run-offs are produced from the T7 promoters by in vitro transcription. c) The labelled RNA is hybridised to a tiling oligonucleotide microarray. Probes that are downstream of a transposon give an "on" signal, other probes give an "off" signal. Note that some probes within an essential gene can give "on" signals, if they can be influenced by transposons outside the gene. Such probes are "non-informative" (see Results and Figure 4). Similarly, probes within a non-essential gene may give an "off" signal if they are upstream of the first transposon.