Aequorea victoria
Several fluorescent protein exist in nature, but the most important one as a research tool is Green Fluorescent Protein (GFP) from the jellyfish "Aequorea victoria", which spontaneously fluoresces upon folding via specific serine-tyrosine-glycine residues. The benefit that GFP and other fluorescent proteins have over organic dyes or quantum dots is that they can be expressed exogenously in cells alone or as a fusion protein, a protein that is created by ligating the fluorescent gene (e.g., GFP) to another gene and whose expression is driven by a housekeeping gene promoter or another specific promoter. This approach allows fluorescent proteins to be used as reporters for any number of biological events, such as sub-cellular localization and expression patterns.