A compound can absorb excitation light entering a fluorescent assay or emission light leaving it, reducing the light available for the measured signal. This inner-filter effect depends on absorption at the relevant wavelengths, compound concentration, and optical path length. The resulting readout distortion can produce an apparent positive or negative result depending on how the assay maps fluorescence to biological activity.

Sources:
- https://www.ncbi.nlm.nih.gov/sites/books/NBK343429/

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A qualitative procedure used to identify compounds with a desired activity at the drug target. The two main categories of assays in drug discovery are biochemical and cell-based assays.

Assay

In a cell-based firefly-luciferase reporter assay, an inhibitory ligand can stabilize the reporter enzyme, allowing more reporter protein to accumulate during incubation. If the detection reagent's substrate conditions reduce competitive inhibition, the accumulated enzyme can produce increased luminescence despite the compound's inhibitory activity against luciferase. This measurement effect can resemble reporter activation without establishing increased transcription.

Sources:
- https://www.ncbi.nlm.nih.gov/sites/books/NBK374281/

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