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:In agreement with other studies (Hakkola et al., 1994; McKinnon et al., 1991; RodriguezAntona et al., 2001; Schweikl et al., 1993), CYP1A2 mRNA expression was detectable in all samples, although it was below the limit of quantitation in two samples.
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