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:Despite evidence derived from human ALS and mouse and cells models of ALS indicating that mitochondria have a role in disease pathogenesis (Beal, 2005; Martin, 2010a; Reddy and Reddy, 2011), recent human ALS clinical trials with putative mitochondrial acting drugs have been unsuccessful.
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