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:TSSs of HERG1b, KCNQ1a, and KCNQ1b genes were determined with Ambion’s RNA-Ready cDNA Human Heart RNA Ligase-Mediated Rapid Amplification of cDNA Ends (5 -RACE) kit, as previously described (15).
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