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:Human serum may be used as a substrate for ESC culture (Xu et al., 2001; Amit et al., 2004; Beattie et al., 2005; Stojkovic et al., 2005a; Fletcher et al., 2006; Braam et al., 2008; Miyazaki et al., 2008; Baxter et al., 2009; Valamehr et al., 2011).
Category: