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 dat-1(ok157) strain was a gift of J. Duerr and J. Rand (Oklahoma Medical Research Foundation) Mutant lines were initially screened using single worm PCR (24, 25) with oligos directed to the gene of interest.
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