The Green Fluorescent Protein (GFP) from the jellyfish Aequorea victoria is used as a fluorescent indicator for monitoring gene expression in a variety of cellular systems, including living organisms and fixed tissues. Unlike other bioluminescent reporters, GFP fluoresces in the absence of substrates, cofactors, or other intrinsic or extrinsic proteins. Purified GFP is a 27kD monomer consisting of 238 amino acids and emits green light (emission maximum at 509nm) when excited with blue or UV light.