A window on hippocampal neurogenesis Addition of new neurons to the adult brain is key to the hippocampal functions of learning and memory. Pilz et al. labeled individual progenitor cells in the mouse hippocampus and watched them in situ for the next 2 months (see the Perspective by Götz). The results revealed the developmental progression as progenitor cells gave rise to mature cells of the dentate gyrus. Science , this issue p. 658 ; see also p. 639