reporter gene
Suppose you want to know when and where a gene switches on inside a living organism, or whether your engineered DNA got into a cell at all. The trouble is that most genes are invisible — their products do nothing you can see. The clever fix is to attach an easy-to-see 'tattletale' gene whose activity you can readily detect. That tattletale is a reporter gene.
A reporter gene encodes a product that is easy to detect and measure — it produces visible colour, light, or fluorescence. You hook the reporter up to the control sequence (the promoter) of the gene you are studying, or fuse it onto a protein of interest. Then whenever that promoter is active, the reporter is made too, and its glow or colour 'reports' the activity. The famous reporters are: GFP (green fluorescent protein) from a jellyfish, which glows green under blue light, letting you watch a gene's activity in living cells; lacZ, whose enzyme turns a colourless substrate blue (the basis of blue-white screening); and luciferase, the firefly enzyme that emits light, allowing extremely sensitive, quantitative measurement.
Reporter genes turned the invisible workings of genes into something you can literally watch and measure. You can track exactly which cells in an embryo turn a gene on, quantify how strongly a promoter fires under different conditions, confirm that a transfection worked, or follow a tagged protein moving around inside a living cell in real time. The 2008 Nobel Prize in Chemistry recognized the discovery and development of GFP, a reagent that transformed cell biology by making the previously invisible glow.
Fuse GFP onto a protein of interest and express it in cells. Under blue light, the protein glows green wherever it goes — so you can watch in a living cell whether your protein heads to the nucleus, the membrane, or elsewhere, in real time, without killing the cell.
GFP makes an otherwise invisible protein glow green in living cells.
A reporter only reports faithfully if the tag does not perturb what it is watching — a bulky reporter fused to a small protein can disturb that protein's folding, location, or function, so controls matter.