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:A number of polymorphisms in UGT1A1 and UGT1A9 affecting expression and protein function have been identified and could potentially modulate the metabolism of SN-38 in vivo (Zheng et al., 2001; Guillemette, 2003; Villeneuve et al., 2003; Girard et al., 2004; Yamanaka et al., 2004), as well as response to irinotecan-based chemotherapy (Ando et al.
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