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 following mutants and transformants have been described previously: ST:YFP (Grebe et al., 2003); BRI1:GFP (Russinova et al., 2004); AUX1:HA (Swarup et al., 2001); AUX1:YFP (Swarup et al., 2004); GNOMM696L-myc (Geldner et al., 2003); and orc (Willemsen et al., 2003).
Category: