To investigate whether intracellular glutamate is preferentially used to exchange cystine into the cell via xCT or for biosynthesis, we detected glutamate in the cell culture medium after GLS1 knockout and found that the knockout of GLS1 resulted in reduced glutamate in the culture medium, but the adding back of either KGA or GAC restored glutamate secretion (Figure S10B,C), suggesting that intracellular glutamate synthesised by KGA or GAC in cancer cells may first be used to exchange cystine for GSH synthesis. The gene discussed is GLS; the disease is cancer.