GBM-associated microglia, a specific type of tissue-resident macrophage in the brain, has been shown to promote the expression of stemness markers in GBM cells, including NANOG, SOX2, ALDH1, OCT3/4, and BMI1. Microglia mostly originate from bone marrow-derived myeloid cells and, in GBM, they can establish a protumoral immune population in the so-called “border niche” [106]. The gene discussed is BMI1; the disease is glioblastoma.