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:…neurotoxin 1-methyl-4-pheyl-1,2,3,6tetrahydropyridine (MPTP) to mimic the key clinical and pathological features of PD in man and subhuman primate species has provided a useful model in which to study treatment strategies for PD (Ballard et al., 1985; Burns et al., 1983; Crossman et al., 1987).
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