Chemical rearrangement in GFP. (A) The structure of green fluorescent protein (GFP). The rearrangement and oxidation of the sequence Ser-Tyr-Gly is the source of fluorescence. (B) Mutants of GFP emit light across the visible spectrum. (C) A melanoma cell line engineered to express one of these GFP mutants, red fluorescent protein (RFP), was then injected into a mouse whose blood vessels express GFP. In this fluorescence micrograph, the formation of new blood vessels (green) in the tumor (red) is readily apparent.

[(A) Drawn from 1GFL.pdb; (B) R.Y. Tsien. Integr. Biol. 2:77–93, 2010, Fig. 12; (C) M. Yang, et al. Proc. Natl. Acad. Sci. U.S.A. 100:14259–14262, 2003, Fig. 2B]