eLife Assessment
This study demonstrates the application of END-seq, originally developed to study genomewide DNA double-strand breaks, to telomere biology; the work packs a punch, concisely demonstrating the utility of this approach and the new insights that can be gained. The authors confirm that telomeres in telomerase-positive cells terminate with 5'-ATC in a Pot1-dependent manner, and demonstrate that this principle holds true in telomerase-negative ALT cells as well. S1-END-seq is similarly developed for telomeres, showing that ALT cells harbor several regions of ssDNA. The study is well-executed and convincing, the new insights are fundamental and compelling, and the optimized END-seq approaches will be widely utilized. The work will prompt additional studies that the reviewers look forward to, including combining telomeric END-seq with long-read sequencing to address the distribution and origin of variant telomere repeats and ssDNA along telomeres in ALT and telomerase-positive settings.