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 gene encoding the spike protein of CoVs is an important gene responsible for viral entry and therefore has been extensively studied (Sun et al., 2008; Belouzard et al, 2012; Li et al, 2012; Wang et al, 2014; Li, 2015; Liu et al, 2015).
Category: