Cover photograph (Copyright © 2000, American Society for Microbiology. All rights reserved.): Coronal section through operated facial nerve nuclei of an axotomized CX3CR1^+/GFP mouse (day 7 after axotomy). Axotomy of the facial motor neuron induces a reaction in the brain stem involving migration, proliferation, and differentiation of microglial cells (green). Heterozygous CX3CR1^+/GFP mice harbor a targeted replacement of one allele of the CX3CR1 chemokine receptor gene by the green fluorescent protein (GFP) gene. Microglial cells expressing CX3CR1 are green fluorescently labeled in CX3CR1^+/GFP mice, allowing a high-resolution analysis of the microglial response to nerve injury. Neuronal nuclei are detected by the anti-neuronal nucleus-specific antibody NeuN (blue). (See related article by Jung et al. in the next issue.)