Immunohistochemical analysis of a panel of 22 patient-derived GBM cell lines grown in vitro under neural stem cell conditions (Fig. S4A) showed that 59% of lines (13/22) exhibited very few (<20% of cells) or no PROM1+ cells and intermediate staining (positivity in 20–50% of cells) in 18% of the lines (4/22). The gene discussed is PROM1; the disease is glioblastoma.