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:The extracellular region of SIRPα interacts with its ligand, CD47, which is also a member of the Ig superfamily,(19,20,24) with such interaction promoting the tyrosine phosphorylation of SIRPα.(20,25) In contrast to the relatively restricted distribution of SIRPα, CD47 is expressed in most cell types including a variety of hematopoietic cells.
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