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:The third group of proteins might be necessary to cope with the alterations in electron Xow such as NdhD2, OCP, and the thioredoxin-like AhpC (Ohkawa et al. 2000; Kobayashi et al. 2004; Wilson et al. 2006).
Category: