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:Upon activin stimulation, Smad2 associates with Smad4, accumulates in the nucleus, and interacts with the DNAbinding protein, FAST-1 or FAST2 (reviewed in Attisano and Wrana, 1998; Kretzschmar and Massagué, 1998; Heldin et al., 1997; Whitman, 1998).
Category: