Genes that code for different colors (GFP = green fluorescent protein, RFP = red, YFP = yellow, CFP = cyan) get put into the DNA of the mice. They also add a gene for a molecule called “Cre recombinase” that randomly cuts out one or more color. So each cell has its own combination of colors that shows up in a microscope. This allows scientists to track one individual neuron to see where it starts and ends. It also makes some pretty awesome images!