The green fluorescent protein (GFP) is a protein that exhibit bright green fluorescence when exposed to blue light. The protein is in the shape of a cylinder, comprising 11 strands of beta-sheet with an alpha-helix inside and short helical segments on the ends of the cylinder. Inward-facing sidechains of the barrel induce specific cyclization reactions in the tripeptide Ser65-Tyr66-Gly67 that lead to chromophore formation. Its amazing ability to generate a highly visible, efficiently emitting internal fluorophore is both intrinsically fascinating and tremendously valuable. The green-fluorescent protein (GFP) of the jellyfish Aequorea victoria has always been used as a universal reporter in a broad range of heterologous living cells and organisms. GFP has become well established as a marker of gene expression and protein targeting in intact cells and organisms.