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:We recently described the promoter of the mouse and human SCN8A genes, which contain a cluster of four mutually exclusive 50 noncoding exons, exon 1a to 1d, each of which is spliced directly to the first coding exon (Drews et al. 2005).
Category: