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:For X-ChIP, we found that muscle chromatin shearing was optimal when the tissue was fixed for 5 min in 1% FA and chromatin was sonicated for 15 min, resulting in DNA fragments ranging from 100 to 600 bp as described previously [6, 28].
Category: