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:that can destroy tumor cells but are instead induced to suppress anti-tumor immune responses by releasing elevated levels of the immune suppressive products, nitric oxide, IL10 and TGF- , and down-regulating levels of the immune stimulatory product IL-12 [10, 17, 14, 20].
Category: