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:To date, the knowledge of mito-nuclear interaction is somewhat limited, although some networks of anterograde and retrograde regulation have been described (Liu and Butow, 2006; Woodson and Chory, 2008).
Category: