The growth of different metastatic lesions within an individual cancer patient is driven by the same genetic mutations, a new study reports. This finding suggests that a biopsy of a single metastatic lesion may provide an oncologist with sufficient information to select the best treatment for a patient with widespread metastatic disease. Treatment decisions for patients diagnosed with a primary tumor are increasingly guided by analysis of the genetic mutations within the tumor that drive its growth. However, metastases can develop in patients months or years after treatment of their primary tumor. Although metastases are responsible for most cancer-related deaths, relatively little is known about driver gene mutations in these lesions. This is in part because primary tumor samples are more readily available and in part because patients presenting with metastatic disease often have been treated with drugs that introduce new mutations, which can complicate analyses. In a new study, Johannes Reiter, Bert Vogelstein and colleagues analyzed 76 untreated metastases from 20 patients with different types of cancer, including breast, colorectal, endometrial, gastric, lung, melanoma, pancreatic, and prostate cancer. They found that different metastatic lesions within a patient share the same driver gene mutations. What's more, the authors showed that the driver gene mutations that were not present in all metastases of a given patient are unlikely to have functional effects. Finally, the authors developed a mathematical model of tumor growth and evolution that helps explain their observations.