We focused on BMI1 because of the growing body of evidence indicating that specific PCGF proteins confer cell type–specific non-overlapping functions to PRC1 complexes (13), with BMI1 (PCGF4) being required for proliferation and self-renewal of NSC (86) while at the same time having a well-documented role in GBM and being associated with a poor prognosis in cancer (20,32,87–90). The gene discussed is BMI1; the disease is cancer.