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:After divergence from chicken, the mammalian lineage acquired three additional 50 noncoding exons that lack
sequence similarity to exon 1c. Exon 1a is derived from LINE element sequence (Drews et al. 2005).
Category: