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:…for multiple bands is that chick brain contains at least two BgtR subtypes because of the expression of a8 subunits (Schoepfer et al., 1990; Gotti et al., 1992, 1994; Keyser et al., 1993). a8 subunits, which are highly homologous to a7 subunits, are not found in mammals (Elgoyhen et al.,…
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