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:subtilis perRBS mutant, the expression level of fur is known to be less than one-third of those of katA, mrgA, and ahpC (Fuangthong et al., 2002; Faulkner et al., 2012).
Category: