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:Piperlongumine (PL) obtained from the fruits and roots of the long pepper plant is a pyridine alkaloid that has been reported to selectively exert cytotoxicity in a wide variety of tumor cell types both in vitro and in vivo, while sparing non-cancerous normal cell types (3).
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