MGMT and Cognitive impairment: However, these drugs cause cognitive impairment due to normal brain cell damage [9,10], and tumors treated with these drugs develop chemo-resistance, largely due to high O6-methylguanine-DNA methyltransferase (MGMT) levels, to inactivation of mismatch repair enzymes MLH1 and MSH29 and over-expression of multidrug resistance proteins [10].