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:The EPIYA motifs have been classified into four types, A, B, C and D (Higashi et al., 2002a), based on the amino acid sequences following each type and all types can be phosphorylated in vitro.
Category: