Gerlinger et al. conducted whole-exome sequencing of biopsy samples taken from different tumour regions from patients with renal cell carcinoma. They showed that the individual tumours displayed extensive intratumour heterogeneity. For example, 63–69% of somatic mutations were not detectable across every tumour region, several tumour suppressor genes showed multiple distinct and spatially separated inactivating mutations within a single tumour, and gene expression signatures of good and poor prognosis were detected in different regions of the same tumour. These findings present challenges to the development of personalized medicine and cancer biomarkers.