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:Previous studies have also reported high expression levels of Cyclin B1 in a variety of cancers such as breast, cervical, gastric, coloectal, colon, and prostate cancers, as well as in myelogenous leukemia cells ([25-28]).
Category: