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:(d) Degradation of 35S-labeled wild-type human cycB1-NT (1-88) in mitotic Xenopus extract supplemented concomitantly with unlabeled WT human cycB1-NT (1-88), WT or D-box mutant sea urchin cyclin B1 NT (13-110), at indicated concentrations.
Category: