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:To date, most of genes involving in HM uptake, transport, sequestration, and detoxification in woody plants were restricted to Populus (Marmiroli et al., 2011; Luo et al., 2016).
Category: