  Classifications of cancer have largely been in service of diagnosis, prognosis, and treatment decisions. Oncologists, surgeons and pathologists typically classify cancer types and subtypes, in light of organ and tissue type of origin (such as leukemia, or glioma), broadly shared signs and symptoms, disease course, gross histopathology (size, shape and rate of division of cells), and “biomarkers” (e.g., estrogen sensitivity, genetic profile). Solid tumors (of the lung, for instance) may be classified as either “benign,” or “malignant,” and malignant cancers can present at different “stages” and have different “grades.” Pathologists use various measures (e.g., size, extent of lymphatic involvement, number of metastases, number of lymphocytes (white blood cells)) to estimate stage of progression of the disease. “Grade” is a term used to classify solid tumors in light of other measures (e.g., degree of “dedifferentiation” of cells in a tumor—or loss of characteristics typical of a breast, prostate, or colon cells and tissue) and is taken to indicate likely disease course. Decisions about staging criteria are conventionally agreed upon by specialists in oncology and pathology, though informed by the best available evidence. Early stage disease can be very difficult to classify, and thus also to know whether (or how) treat. And, some cancers seem particularly recalcitrant to treatment. This led researchers to ask: Are there distinctive genetic or molecular “signatures” of each cancer that can help us estimate risk of progression, or chances of recurrence, as well as predict responses to treatment? Perhaps these molecular signatures are the key to a natural taxonomy of cancer types and subtypes?