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:These enzymes have been implicated in pesticide resistance in plants and insects (Sheehan et al., 2001), and
some GST polymorphisms are thought to alter cancer susceptibility in mammals (Hayes and Pulford, 1995).
Category: