Tsien and colleagues published the crystal structure of the 238 A.A. long green fluorescent protein (GFP).
With this data, the authors could determine what had to be modified within the protein in such a way that the protein would be able to absorb and emit light in other areas of the spectrum, i.e. these protein variants of GFP would not only shine much stronger, but in quite a few different colors such as cyan, blue, and yellow, eventually expanding the color palette to red and orange, etc. The variants also became more photostable, and excited at a wavelength that matched that of conventional microscopes filter sets, remarkably increasing the practical useability of GFP. The authors created a powerful protein tool that allowed researchers to perform imaging experiments that could easily discriminate between and follow multiple tagged proteins in cells and higher organisms. Order of authorship in the original publication: Ormö, Cubitt, Kallio, et al, Tsien, Remington.

In 1998 Tsien authored a much longer report on GFP: "The green fluorescent protein," Ann. Rev. Biochem., 67, 509-544.

In 2008 Tsien shared the Nobel Prize in Chemistry with Osamu Shimomura, Martin Chalfie  "for the discovery and development of the green fluorescent protein, GFP."

(Thanks to Juan Weiss for this reference and its interpretation.)