Telomeres, the structures capping the ends of chromosomes, play a vital role in the maintenance of genomic integrity. Here, Christopher Fegan, MB, MD, FRCP, FRCPath, of Cardiff University, Cardiff, UK, introduces telomere length as a key determinant of telomere function, before presenting exciting data that suggests telomere length is a powerful predictor of outcome following fludarabine, cyclophosphamide and rituximab (FCR)-based treatment in chronic lymphocytic leukemia (CLL) patients. Speaking at the American Society of Hematology (ASH) 2018 Annual Meeting and Exposition in San Diego, CA, Prof. Fegan thinks telomere length could be used to inform the design of future clinical trials.