CDKN2A and glioblastoma: 110 As already described, Olig2, a marker of oligodendroglia precursors, is expressed in 100% adult gliomas111 and 100% of CD133 + glioma stem cells. Olig2 is required for tumor formation in p16Ink4a/p19ARF double null murine gliomas. Recent studies have confirmed the extensively studied role of members of the TGF-β superfamily in morphogenesis and specifically brain development.112 As described above, TGF-β may play a significant role in GBM development as well.