GSH functions as a crucial cofactor for the activity of GPX4 enzyme in eliminating ROS, whereas cysteine is the rate-limiting precursor required for GSH synthesis.[49] Generally, cancer cells take up extracellular cysteine through the cystine-glutamate antiporter system xc-, which facilitates the uptake of cystine (the oxidized dimer of cysteine). This evidence concerns the gene GPX4 and cancer.