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:Cysteine to serine mutation of WhiB1 was generated by overlapping extension PCR using a primer pair consisting of wild type [23] and appropriately mutated primer as mentioned in the Table 2 (Additional file 2).
Category: