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 defects of yeast ubc9-1
mutants in the degradation of B-type cyclins may also indicate a link between SUMOylation and ubiquitin-mediated proteolysis (Seufert et al
., 1995).
Category: