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Text:Buck, unpublished observations), nitric oxide synthase expression and activity resulting in the synthesis of NO, which interacts with superoxide to generate peroxynitrite (33, 48), a compound highly reactive with sulfur-containing amino acids, such as Cys (12).
Category: