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:ORL1, a recent addition to the opioid receptor family, is the only opioid receptor subtype selectively activated by OFQ/N, an endogenous opioid ligand (Meunier et al., 1995; Reinscheid et al., 1995).
Category: