Another powerful use of GFP is to express the protein in small sets of specific cells. This allows researchers to optically detect specific types of cells "in vitro" (in a dish), or even "in vivo" (in the living organism). Genetically combining several spectral variants of GFP is a useful trick for the analysis of brain circuitry (Brainbow). Other interesting uses of fluorescent proteins in the literature include using FPs as sensors of neuron membrane potential, tracking of AMPA receptors on cell membranes, viral entry and the infection of individual influenza viruses and lentiviral viruses, etc.