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:…of Ubc9 and Smt3 arrest with short spindles and high levels of Pds1
It was reported previously that yeast ubc9 D cells, which were kept alive by UBC9
expressed from the galactoseinducible GAL1-10
promoter, arrested in mitosis upon promoter shut-off in glucose medium (Seufert et al
., 1995).
Category: