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:CXCR4 by various receptor tyrosine kinases has been linked to both normal and disease-related signaling in multiple cell types.(16-19) This transactivation often involves a physical association between receptors and site-specific phosphorylation of CXCR4 that activates various signaling outcomes.
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