Temozolomide is an antitumor, antiangiogenic compound, that has been shown to work by alkylating DNA; typically through addition of a methyl group to guanine in genomic DNA. The efficacy of this compound is dependent on its ability to methylate the DNA, thus triggering apoptosis; however, some cells are able to repair this damage via the enzymes MGMT (O-6-methylguanine-DNA methyltransferase). As a consequence Temozolomide is far more efficacious in tumor cells that have silenced this repair enzyme. Research has also been conducted to determine whether MGMT inhibitors such as O-6-benzylguanine might help to increase the efficacy of Telozolomide. Furthermore, Temozolomide has been implicated in causing arrest at the G2-M cell cycle checkpoint, which might also play a role in its cytotoxicity.