As a student, Tsien was fascinated with fluorescence resonance energy transfer, a distant dependent process in which a dye molecule transfers its energy to a nearby dye molecule, and fluoresces. He thought this might be a useful way to create a fluorescent sensor that would indicate when a particular biochemical reaction was occurring inside cells. His idea was to attach a dye molecule to the protein and a catalyst of the reaction. When the reaction was taking place, the two should be very close to each other and fluoresce. After much work on mutating the Green Fluorescent Protien (GFP) and learning how to attach it to cells, Tsien and his collaborators were able to make important biochemical processes visible.