In their words:
Neurons in different brain regions perform different functions and must communicate with each other for the brain to process information and control behavior. Neurons communicate across long distances by sending axons far away from their cell bodies. We are interested in discovering which neurons communicate with particular brain regions so we designed a technique identify them.
We used a type of mouse where cells fluoresce red in the presence of a protein called cre recombinase. Since cre is not a native protein in the brain, none of the neurons glow until we add it. We built a genetically altered virus to deliver the gene for cre to particular neurons by infecting their axons. By making a small virus injection into one brain area, we can light up the cell bodies of neurons communicating with that area from far away.