Rather than being a reporter enzyme, green fluorescent protein (GFP) (and its engineered derivatives) is particularly suited to bioimaging assays in living cells or live animals. GFP is produced by the jellyfish, Aequorea victoria. Upon excitation by UV light, GFP emits green light with an emission maximum at 509 nm [4] . The intensity of light emissions has been improved by protein engineering in the fluorophore region of the protein. GFP-based reporter assays provide a distinct advantage because they do not require cell permeabilization or the addition of exogenous substrates. Therefore, these proteins are suitable for drug screening in HTS assays. Recent improvements in GFP detection technologies allow changes in the localization of GFP-tagged proteins, in response to small molecules, to be detected at the subcellular level in living cells cultured in 384-well plates. Therefore, GFP is suitable as both a reporter of transcriptional activation, and of functional protein translocation in the cell, in response to various stimuli.