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 over-expression of EGFR (erbB-1 receptor) is seen in 20% to 81% of breast tumours, whereas HER2 receptor overexpression is seen in approximately 20% [1-4].
Category: