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:GUS Assays
 Qualitative histochemical GUS staining of seedling roots was performed as previously described (Hung et al., 1998; Wang et al., 2002).
 qRT-PCR Assays
 For qRT-PCR, total RNAwas extracted from root tips of 5-d-old seedlings using the RNeasy plant mini kit (Qiagen).
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