XRCC6P5 and neoplasm: These agents are cytotoxic principally by virtue of their ability to alkylate DNA at the O6 position of guanine, and there is now considerable evidence that the DNA repair protein O6-alkylguanine-DNA alkyltransferase (MGMT) plays a key role in determining tumour resistance to these drugs (Margison and Santibáñez Koref, 2002; Gerson, 2002, 2004).