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 the T99 mutant was rendered unable to grow on 4-CP by an insertion of a transposon into a hydroxyquinol 1,2-dioxygenase gene, the results suggest that cleavage of hydroxyquinol is a vital part of degradation of 4-NP and 4-BP as well as of 4-CP (Nordin et al. 2005).
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