For more than a century, pathologists have classified and graded brain tumors, examining samples under a microscope and looking for telltale signs of tumor class and biological aggressiveness such as cell morphology, cell death, new blood vessels and cell proliferation. However, the advent of more affordable genetic sequencing and the discoveries from large studies such as The Cancer Genome Atlas (TCGA) have upended this paradigm, allowing classification and grading based on more accurate—and less visible —genetic biomarkers.