green fluorescent protein (GFP)

A certain jellyfish glows green in the dark ocean, thanks to a single protein it makes. In the 1990s, biologists realised they could borrow that protein as a built-in flashlight: if you splice its gene onto the gene for any protein you want to study, the cell builds your protein with a tiny green lamp attached. Suddenly you could watch a specific protein in a living cell, in real time, without killing or staining anything. This protein is green fluorescent protein, or GFP.

GFP is special because it glows all by itself: the part that produces the green light forms spontaneously from the protein's own folded structure, needing no added dye or helper molecule. Researchers attach GFP's gene to the gene of a target protein, creating a fusion gene; the cell then makes the target protein carrying GFP as a glowing tag. Shine blue light on the cell and the tagged protein shines back green, revealing exactly where it goes and when. A whole rainbow of colour variants (blue, cyan, yellow, red) was later engineered so several proteins can be tracked at once.

GFP transformed biology by making the invisible movements of life directly visible in living cells, and its discovery and development won the 2008 Nobel Prize in Chemistry. Unlike immunostaining, GFP works in living cells over time. Its caveat: GFP is a sizeable add-on bolted to your protein, and in rare cases this tag can change how the protein folds, moves, or behaves — so a tagged protein is a very good stand-in for the real thing, but not always a perfect one.