For years we have known that tumours are very heterogeneous, but quantifying the varied molecular composition of tumours has been difficult. Using a single-cell RNA sequencing technique to profile 430 cells from five primary glioblastoma samples, researchers have now quantified the high level of variation in the expression of diverse transcriptional outputs related to signalling, cell proliferation, immune response and hypoxia. A high degree of variability of these transcriptional classifiers was noted across the different cells, highlighting the importance and underappreciated prognostic implications that intratumoural heterogeneity has in terms of glioblastoma biology, prognosis and the treatment of this disease.