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:The PIWI domain in both Ct-Piwi1 and Ct-Piwi2 is located near the 3’ end of the ORF, which is typical of piwi genes [25].
Category: