= Enhancer trap
An \b[enhancer trap] is a <reporter gene> arrangement used to reveal regulatory activity near its genomic insertion site. A weak <promoter> drives a measurable reporter; nearby <enhancers> can alter its expression in particular <cells> or developmental stages. The observed pattern therefore turns otherwise invisible <gene regulation> into a measurable output. It does not require the experimenter to begin with a visible mutant phenotype.<Detection in situ of genomic regulatory elements in Drosophila by C. J. O'Kane and W. J. Gehring>[Abstract, PNAS 84 (1987), pp. 9123–9127; DOI 10.1073/pnas.84.24.9123.]{ref}
In the original <Drosophila> study by <C. J. O'Kane> and <W. J. Gehring>, the reporter encoded <beta-galactosidase>. About 70% of 49 transformed lines showed spatially regulated expression in embryos, including patterns in the <nervous system>. The informative comparison is between insertion contexts: the reporter acts as a sensor of the regulatory environment rather than as a direct readout of one preselected native <gene>.<Detection in situ of genomic regulatory elements in Drosophila by C. J. O'Kane and W. J. Gehring>[Abstract: reporter identity, number of lines and embryonic expression patterns.]{ref}
For an illustrative interpretation, reporter activity in one tissue and not another suggests a difference in the regulatory inputs available there. It does not alone identify the responsible <enhancer>, prove its exact native target, or establish that every native expression feature has been reproduced. Those questions require additional evidence.
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