It has been known for some time that telomeres get shorter with each round of DNA replication. To prevent this, highly proliferative cell types elongate their telomeres using the reverse transcriptase telomerase. On p. 4059,Ferrón et al. investigate the effect of telomere erosion on the proliferation of neural stem cells (NSCs). Using mice that lack functional telomerase, they show that progressive telomere shortening impairs the proliferation of adult NSCs in vitro. Surprisingly, however, telomere erosion does not affect the proliferation of embryonic NSCs, despite their accumulation of chromosomal abnormalities. The researchers conclude that adult and embryonic NSCs have very different responses to telomere shortening, and that some types of stem cell can bypass DNA damage checkpoints. They point out that by understanding the intracellular mechanisms that regulate NSC cycling,techniques for activating neurogenesis might emerge for use in treating brain disease.