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 Illumina platform and Roche GS-FLX are an effective combination to call SNPs when lacking a reference genome [17–20], but additional labor, time and cost are required to build a rough reference with GS-FLX.
Category: