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:There is a gradual increase in the double phosphorylation of the Thr-Glu-Tyr motif, characteristic of activated kinases such as ERK1, 2 and 5 in two proteins of 80 and 105 kDa in capacitating human spermatozoa (de Lamirande and Gagnon, 2002; Thundathil et al., 2002, 2003).
Category: