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:Because the mammalian CSC does not bind tightly to the PI3P-binding sorting nexins, it is unclear what prevents the sorting nexins from driving the formation of empty tubules devoid of cargo (Cullen and Korswagen, 2012).
Category: