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:Besides, the cysteine-maleimide linkages are instable in a circulatory system, because the maleimide-linker reaction can be exchanged by the active thiol group of cysteine in human serum, resulting in the premature release of toxins before internalization [7].
Category: