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:Concerning heavy metals tolerance and accumulation in plants, many genes involved in adaptive tolerance and detoxification of HMs have been identified and isolated from several plants (Verbruggen et al., 2009; Krämer, 2010; Ueno et al., 2011; Luo et al., 2016).
Category: