The green fluorescent protein (GFP) from the jellyfish Aequorea victoria has recently leapt to prominence within numerous biological fields. Interest in the use of GFP has grown enormously over the past 5 years. The number of papers concerning GFP held in the Web of Science database, which abstracts the scientific literature (http://wos.mimas.ac.uk), rose from 13 in the period 1981–1994, to over 3400 in the period 1995 to October 2000. This is due to the ability to clone and heterologously express GFP genes in a diverse range of cells and organisms, from bacteria and yeast, to plants and mammals, coupled with favorable properties such as high stability, minimal toxicity, noninvasive detection, and the ability to generate the highly visible fluorophore in vivo in the absence of external cofactors. Thus, GFP has become a truly versatile marker for visualizing physiological processes, monitoring subcel