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:The involvement of the clock in Pai-1 gene regulation has been described (Maemura et al., 2000; Minami et al., 2002; Schoenhard et al., 2003; Oishi et al., 2005a; Chong et al., 2006; Oishi et al., 2007), although the precise molecular mechanism on how the clock regulates the intrinsic daily expression of Pai-1 transcription is still unclear.
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