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:However, the amounts of the other PCR cycle enzymes and of electron transport components also vary in parallel with the amount of RuBisCO (von Caemmerer and Farquhar 1981; Makino et al. 1983).
Category: