One of the many useful cell biological techniques of the past few years has been the development of green fluorescent protein (GFP) as a reporter for gene activity, in which the gene of interest and the GFP gene are expressed together. Thus, the appearance of GFP fluorescence can be used to visualize both the expression of the other gene and its cellular location. Although GFP variants exist with different emission wavelengths, more variety is needed to permit multi-fluorescent imaging, or to overcome problems of inherent tissue fluorescence. The recent publication of the atomic resolution structure of red fluorescent protein (RFP) by Mark Wall and colleagues [Nat. Struct. Biol. (2000) 7, 1133–1138] should permit engineering of its fluorescent properties so that FPs can be even more useful as gene-reporters. NC