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 small proportion of 50-RACE products containing exon 1c that we previously reported for human and mouse was an underestimate caused by the difficulty of amplification of this 93% GC fragment (Drews et al. 2005).
Category: