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:PCR-targeted mutagenesis was used to replace precisely the cmdABCDEF or SAV4098-4103 genes with an antibiotic resistant gene and then remove the marker but leaving an 81-bp "scar" sequence when necessary [20].
Category: