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:…FAST2 o form an ARF (Chen et al., 1997; Labbé et al., 1998; Zhou et al., 1998) which acts as a transcriptional activator to regulate gene expression (Liu et al., 1997; Labbé et al., 1998; reviewed in Attisano and Wrana, 1998; Kretzschmar and Massagué, 1998; Heldin et al., 1997; Whitman, 1998).
Category: