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:Several tyrosine kinase receptors transactivate CXCR4 in order to mediate signal transduction in various cell types.(16-19) Here, we asked whether ligation of TCR induces an association between TCR and CXCR4 and/or transactivation of CXCR4 in order to mediate cytokine production.
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