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:This could be explained by the requirement for specific spacers that match the invader DNA (Barrangou et al., 2007; Brouns et al., 2008; Marraffini & Sontheimer, 2008), and also by the existence in E.
Category: