Clinical studies using methionine restriction are limited and more complicated to control than rodent studies; however, dietary methionine restriction together with the use of chloroethyl nitrosourea chemotherapy in patients with recurrent glioma or metastatic melanoma reduced O-6-methylguanine-DNA methyltransferase (MGMT) activity, which is a main mechanism of nitrosourea resistance [117]. Here, MGMT is linked to central nervous system cancer.