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:In both cases, UV irradiation generates short-lived (μsec range) radicals that react with amino acids in close proximity (zero distance) forming covalent bonds (Pashev et al. 1991; Hafner et al. 2010a).
Category: