FIGURE 2 Neuronal networks can be traced by turning the network into a brainbow. Transgenic mice with genes encoding GFP variants and different (modified) intervening lox sites are mated to transgenic mice with the gene encoding the Cre recombinase, leading to heterozygous progeny that carry out Cre-mediated recombination in developing neurons. (a) The different lox sites differ in their core sequences, so lox1 reacts only with lox1, and lox2 only with lox2. (b) In this case, the cassette utilizes three or more different lox sites. Cre-mediated recombination results in different patterns of GFP variant expression, and thus a different color, in each neuron. (c) The resulting brainbow of neurons, visualized by a specialized form of light microscopy (epifluorescence).