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:All katanin derivatives were PCR amplified from the human p60 (McNally and Thomas, 1998) or human p80 (Hartman et al., 1998) cDNAs previously described.
p80 derivatives The WD40 domain of p80 (WD) consisted of amino acids 1-304, WDpro consisted of residues 1-484, pro-con80 consisted of residues…
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