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:…disulfide bonds formed by conserved pairs of Cys residues not only play a role in maintaining the 3D protein structure but also contribute to the bioactivity of PRL, e.g., the mitogenic activity in NB2 cells (Luck et al., 1992) and osmoregulatory ability in urinary bladder (Doneen et al., 1979).
Category: