To address this question, we compared single nuclei profiles from NeuN-negative cortical nuclei in healthy, non-tumor, and tumor brain tissue and show that non-tumor glia harbor only low-frequency, mosaic aneuploidies, most notably involving chromosome 16p, while GBM-defining (+7/−10) are restricted to tumors. The gene discussed is RBFOX3; the disease is glioblastoma.