Telomeres are repetitive DNA sequences at the ends of linear chromosomes that are required for chromosome maintenance and genome stability. In proliferating cells, a mechanism that involves telomerase and the DNA replication machinery maintains telomeres. In the absence of telomerase, progressive telomere shortening eventually triggers cell senescence; this protective mechanism is deregulated in cancers. Interestingly, most eukaryotic telomeres can be transcribed to generate a telomeric repeat-containing RNA (TERRA) that persists as a heterogeneous nuclear RNA. Here, Paul Lieberman, Nadia Dahmane and colleagues (p. 4383) examine TERRA expression in a mouse model of medulloblastoma and in human cancer biopsies to gain insights into TERRA function and regulation. In the medulloblastoma model, TERRA accumulates and forms nuclear foci (TERFs) in rapidly proliferating tumour cells, but not in adjacent normal cells. Notably, TERRA aggregates are also present in highly proliferating progenitor neurons in developing mouse brains. Moreover, TERRA expression is elevated in various human cancer tissues. Together, these results show that TERRA accumulates and forms TERFs in highly proliferating normal and cancer cells, and suggest that TERFs could provide a biomarker for cancer-associated telomere dysfunction.