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:Indeed, mammalian nuclear receptors such as PPAR-a and PPAR-c are activated by fatty acids or their derivatives [26,27] and structural analysis of the ligand binding domain of HNF4a revealed long chain fatty acids bound in the ligand binding pocket [35,36].
Category: