 Enhancer trap

An 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.^[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.^[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.