CD44 and glioblastoma: Due to the pronounced heterogeneity of GBMs, many additional cell surface markers besides CD133 [7,13,14] have been proposed to identify GBM stem cells (GSCs), including CD15 (SSEA–1), CD44, A2B5 epitope, integrin α6, CD90, L1CAM, CXCR4, CD44+ID1 (reviewed in [15]).