GFP is a naturally fluorescent protein discovered by chance in the 1960s by Shimomura et al. GFP makes green light out of aequorin's blue light. The chromophore is a modified Ser-Tyr-Gly sequence. GFP can emit light once this short tripeptide has adopted a cyclic conformation with the help of a oxygen. BIOPHYSICOCHEMICAL PROPERTIES: Excitation max (nm): 488; Emission max (nm): 509; Extinction coefficient (Cm-1M-1): 61000. Fluorescent proteins have become a useful and ubiquitous tool for making chimeric proteins where they function as a fluorescent protein tag. Typically they tolerate N- and C-terminal fusion to a broad variety of proteins. They have been expressed in most known cell types and are used as a noninvasive fluorescent marker in living cells and organisms. They enable a wide range of applications where they have functioned as a cell lineage tracer reporter of gene expression or as a measure of protein-protein interactions.