Jeff Lichtman and his co-workers have created a series of transgenic mice that express fluorescent proteins in their neurons. By mating a female expressing blue fluorescent protein in her neurons with a male expressing GFP they could watch differently colored neurons competing for the muscle surface in their offspring, see below.

A mouse with different neuron colors inherited from its two parents can reveal how neurons network in the brain. ( J. W. Lichtman/Washington University School of Medicine)

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