PST3.1a is a selective inhibitor of Mannoside acetyl glucosaminyltransferase 5 (MGAT5), used for studying glioblastoma multiforme (GBM). PST3.1a inhibits TGF-betaR and FAK signalling associated with doublecortin (DCX), increases OLIG2 expression, and suppresses the invasion and proliferation of GBM initiator cells (GICs).