Nature, Published online: 23 April 2025; doi:10.1038/d41586-025-01170-4Reconstructions of the evolutionary relationships among blood-cell clones from individuals with the blood cancer chronic myeloid leukaemia reveal that the cancer clone grows exceptionally quickly, reaching rates of between 10,000% and 1,000,000% per year. This variability in growth rate could explain differences in responses to cancer treatments.