GFP is an Autofluorescent Protein

The core biochemical principle that protein structure determines protein
function is illustrated by green fluorescent protein (GFP), which
is an autofluorescent protein isolated from the Pacific Northwest jellyfish
Aequorea victoria. The chromophore forms spontaneously through
a chemical reaction involving three amino acid residues within the GFP
polypeptide without requiring an exogenous cofactor. Researchers discovered
that the amino acid sequence of GFP is necessary and sufficient
to specify the fluorescent properties of the protein, which is analogous to
how the structure of the DNA double helix is necessary and sufficient
to specify genetic inheritance. The use of GFP as a fluorescent marker
in live cells has allowed cell biologists and physiologists to study cellular
processes in real time.