Green fluorescent protein (GFP) isolated from jellyfish Aequorea aequorea is a 238 amino acid protein with an apparent molecular weight of about 27-30 kDa on SDS-PAGE. Its chromophore is formed by cyclisation and oxidation of the three amino acids Ser65, Tyr66, and Gly67. The numerous applications include, using GFP as a reporter for gene expression, as a marker to study cell lineage during development and as a tag to localize proteins in living cells. Other applications of GFP include assessment of protein protein interactions through the yeast two hybrid system and measurement of distance between proteins through fluorescence energy transfer (FRET) protocols. GFP technology has considerably contributed to a greater understanding of cellular physiology. Energy-transfer acceptor. Its role is to transduce the blue chemiluminescence of the protein aequorin into green fluorescent light by energy transfer. Fluoresces in vivo upon receiving energy from the Ca2+-activated photoprotein aequorin.