Genetically engineered "brainbow” mice express random combinations of cyan, yellow, and red fluorescent proteins in nerve cells. As a result, neurons and their projections are labeled with a multitude of distinct hues. In this image from a portion of the cerebellum, a brain area involved in movement and balance, the multicolor labeling reveals the intricate meshwork created by long neural projections that connect to cerebellar neurons. (These connections, also called synapses, are seen here as large spots.)