To show that glutamate release occurred through system xc−, we incubated an identical number of slices from the same animals in a bath containing 250 μM SAS. SAS-treated slices released significantly less glutamate (1 h: 0.7984 ± 0.15; 2 h: 2.199 ± 0.36) (**P < 0.01) (Fig. 1e). In sham slices, we found that glutamate release was unaffected by SAS, suggesting that system xc− does not contribute substantially to glutamate release in tumor-free brain.