MGMT and glioma: The mechanism of TMZ resistance reported so far is related to the heterogeneity of glioma cells, upregulation of O6-methylguanine DNA methyltransferase (MGMT), DNA repair, and signal transducer and activator of transcription 3 (STAT3) (Pajonk et al., 2010; Happold et al., 2012; Kohsaka et al., 2012).