Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Another potentially important link between oxidative stress and excitotoxicity is the microglial antiporter system Xc 2 (Sato et al., 2002), which releases cytotoxic levels of glutamate into the interstitial compartment to replenish intracellular cystine to maintain the glutathione system, which is challenged by oxidative stress (Qin et al.
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