How are complex processes such as sensory perception, regeneration of nervous tissue or the activation of the immune system during autoimmune disease orchestrated within the living organism? Although these questions appear quite different, fluorescent proteins offer tailor-made tools for these research areas. Scientists enhance their skills in designing and using these indicator proteins – from bacteria to neurons in mice. The design and properties of the proteins can be modified so that they show e.g. biochemical signal cascades or the firing of action potentials in neurons.