Cover legend: Dentate granule cells generated in the adult mouse hippocampus labeled by retroviral expression of green fluorescent protein. This pseudocolored image was obtained by two-photon microscopy in 400-μm-thick sections. Artwork by M. Georgina Davies and Nicolás A. Morgenstern. For more information, see the article by Piatti et al. in this issue (pages 7715–7728).