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:Following the discovery of the receptors, fatty acid endogenous ligands, such as anandamide and 2-arachidonoyl glycerol (2-AG), and a degradation system including a re-uptake mechanism and hydrolytic enzymes have been identi®ed (Devane et al., 1992; Deutsch and Chin, 1993; Mechoulam et al., 1995; Dinh et al., 2002).
Category: