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 Marsupenaeus japonicas and Penaeus semisulcatus, the CHH from eyestalk is involved in the suppression of vitellogenin (Vg) gene expression (Khayat et al., 1998; Tsutsui et al., 2005).
Category: