The Green Fluorescent Protein (GFP) was originally identified as a protein involved in the bioluminescence, which is from the jellyfish Aequorea Victoria. It is widely 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 without the need for exogenous substrates or cofactors, or other intrinsic or extrinsic proteins, making GFP a useful tool for monitoring gene expression and protein localization in vivo. Purified GFP is a 27 kD monomer consisting of 238 amino acids and emits green light (emission maximum at 509 nm) when excited with blue or UV light.