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:Results are expressed as percentage of control (OD at 405 nm measured by cell ELISA of unstimulated cells), arbitrarily set at 100%.
cell line (Wuthrich and Sekar, 1993).
Category: