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:chlorophenolicus T99, a mutant with a transposon insertion in a gene encoding a hydroxyquinol dioxygenase, resulting in an inability to grow on or degrade 4-CP (Nordin et al. 2005).
Category: