PROM1 and glioblastoma: Furthermore, Olig2 can easily be studied by IHC as we previously demonstrated in a small cohort of GBM.12 Olig2 is also a surrogate marker of the PN subtype of GSC, used instead of CD133 as this key PN marker can be studied by FACS but not by IHC.17, 21, 26 The PN phenotype seems to be predominant at the tumor edge based on gene expression profile.17, 21, 45, 46 However, in the present study, the nuclear labeling of Olig2 in the tumor core of initial surgery samples was found in majority of the cases (n = 71/72) with high nuclear expression (≥30%, n = 52/72).