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:Subtype classification was carried out based on estrogen receptor (ER), progesterone receptor (PR), and HER2; C1% of cells being positive for ER and/or PR was defined as ‚Äòhormone receptor (HR) positive,‚Äô while HER2-overexpression was determined according to the 2007 ASCO/ CAP guidelines [14].
Category: