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:We have previously shown that turkey livers possess orthologs to human P450 1A2 and 3A4 which efficiently bioactivate AFB1 to exo-AFBO (Rawal et al., 2010b, 2009; Klein et al., 2000; Yip and Coulombe, 2006).
Category: