This image shows the differential distributions of endogenous mouse (yellow) and exogenous human tau (blue) in the hippocampal dentate gyrus of a transgenic mouse. Endogenous tau is abundantly expressed in mossy fiber axons, the cell bodies of newborn neurons in the inner granule cell layer, and in oligodendrocytes, but the cell bodies of mature granule cells are void of immunolabeling. In contrast, human tau expressed under an ectopic promoter is abundant in the cell bodies of mature granule neurons, but not in newborn neurons. Human tau mislocalizes to the cell bodies when the expression is maintained in mature neurons. This could explain how tau forms pathological inclusions in cell bodies in neurodegenerative disorders. For more information, see the article by Kubo et al. (pages 6781–6797).