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:2, which can inhibit mDA neuronal differentiation Fgf8, Shh and Otx2, genes involved in the early regional specification, are capable of promoting dopaminergic cell specification in greater abundance in their normal location as well as ectopically in naïve hindbrain or forebrain tissue [13,21,25,28,32].
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