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:Previously, a phosphoproteomic study demonstrated that AbfR1 is phosphorylated in vivo at a tyrosine and serine residue, Y84 and S87 (Reimann et al., 2013).
Category: