The nuclei of neural stem and progenitor cells are marked by the expression of nuclear-localized cyan florescent protein (CFP). In this issue, Amelchenko et al. present a novel behavioral task and show that ablation of neural stem cells using gamma-irradiation, while sparing learning and memory, selectively impairs cognitive flexibility. They also show that distinct components of the task activate cohorts of newly generated neurons that were born three months earlier, suggesting a significantly extended critical period during which adult-born neurons exhibit augmented plasticity. For more information, see the article by Amelchenko et al. (pages 6061–6083). Cover image: Grigori Enikolopov.