CD44 and glioblastoma: Diverse GBM cell populations defined phenotypically, based on the expression of markers such as CD133, CD44, ITGB8, PTPRZ1, or SOX2 [8,9,10,11,12,13,14,15,16], or based on fundamental functional characteristics, including being slow-cycling [6,7,17], exhibit the hallmark traits of CSCs described above.