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:…as sole carbon source, the T99 mutant strain, with a disrupted hydroxyquinol dioxygenase gene, behaved the same way as when challenged with 4-CP (Nordin et al. 2005), i.e., no growth occurred and an orange pigment accumulated in the cultures, suggesting that the same catabolic route was used…
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