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:Moreover, a neuroprotective role of estrogen in the nigrostriatal dopaminergic system is well supported by in vitro and in vivo studies using different toxin models, namely 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) [6, 7] and 6-hydroxydopamine (6-OHDA) [8-10].
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