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:Phosphoproteomic studies of Arabidopsis plasma membrane proteins (Nühse et al., 2004) indicate that at least two PHT1 transporters (PHT1;1 and PHT1;4) are phosphorylated (for PHT1;1 modification affect Ser-520).
Category: