Even very aggressive types of cancer can usually be treated successfully if detected early enough. For example, melanoma is a type of skin cancer that is extremely aggressive yet reasonably easy to treat because it is often detected early by visual exam. In contrast, tumors that arise in the internal organs are often detected at a later stage and are therefore much more difficult to treat. For this reason, there is a great deal of research on the development of noninvasive diagnostic tests that detect cancer at its earliest stages. In a recent study published in Nature Genetics, researchers from the University of California, San Diego describe a test that could not only detect tumor-derived DNA in the blood, but also determine its tissue-of-origin by analyzing patterns of DNA methylation. Much work remains to be done to perfect such tests, but their development could lead to a simple, universal screen for cancer that could be administered prior to symptoms developing.