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:…differentially expressed during in vitro xylem differentiation in Arabidopsis suspension cells showed that AtGATL6 and AtGATL9 expression levels decreased rapidly before xylem vessel element formation (Kubo et al., 2005), further supporting a role for these two genes in primary cell wall synthesis.
Category: