There are many techniques to utilize GFP in a live cell imaging experiment. The most direct way of utilizing GFP is to directly attach it to a protein of interest. For example, GFP can be included in a plasmid expressing other genes to indicate a successful transfection of a gene of interest. Another method is to use a GFP that contains a mutation where the fluorescence will change from green to yellow over time, which is referred to as a fluorescent timer. With the fluorescent timer, researchers can study the state of protein production such as recently activated, continuously activated, or recently deactivated based on the color reported by the fluorescent protein. In yet another example, scientist have modified GFP to become active only after exposure to irradiation giving researchers a tool to selectively activate certain portions of a cell and observe where proteins tagged with the GFP move from the starting location. These are only two examples in a burgeoning field of fluorescent microcopy and a more complete review of biosensors utilizing GFP and other fluorescent proteins can be found here