Separate parts of a single tumour can contain distinct genetic mutations, and clinicians might suggest contradicting prognoses or treatments on the basis of biopsies from different areas. Charles Swanton at Cancer Research UK's London Research Institute and his colleagues analysed this diversity by reconstructing the evolution of four kidney cancers, sampling several parts of each primary tumour, as well as secondary ones from organs to which the cancer had spread.

In one patient, only one-third of the total mutations were found in all the samples, and one-quarter were unique to just one. This has implications for biomarker analysis and cancer-genome studies, which rely on single biopsies. It might also explain why many cancer treatments eventually stop working: they affect only part of a tumour, allowing the remainder to bounce back.

N. Engl. J. Med. 366, 883–892 (2012)