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:Changes in the level of gene expression in cancer (4, 8, 12, 13) and aberrant expression of tissue-restricted gene products in cancer (14, 15) are factors in the development of a humoral immune response in cancer patients.
Category: