Recently the green fluorescent protein (GFP) from the jellyfish Aequorea victoria has emerged. It has been widely studied and used in biochemistry and cell biology. The protein emits green light (λ_max＝508 nm) when excited with ultraviolet (UV) or blue light (λ_max＝395 nm, minor peak at 479 nm). It is extremely useful as a marker for monitoring gene expression and as a tag in studying protein localization in organisms, intact cells and organelle. Mutagenesis and engineering of GFP fused into chimeric proteins are opening new prospect in physiological indicators, biosensors, photo-chemistry and production of luminescent fiber.