This maximum intensity projection confocal image shows a neurobiotin-filled neuron (yellow) in a coronal brain slice containing the dorsal raphe nucleus from an adult mouse. Unlike the surrounding neurons, the neurobiotin-filled neuron does not express tryptophan hydroxylase (a marker of serotonin neurons, blue). For more information, see the article by Khamma et al. (pages 968–979). Cover image: J. Khamma and S. Gantz.