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 image on the right.

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)