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:Among several endogenous lipophilic molecules known to activate CBRs, the best characterized are N-arachidonoylethanolamine (anandamide, AEA) and 2-arachidonoylglycerol (2-AG), which are known to modulate synaptic transmission throughout the brain (Zogopoulos et al., 2013).
Category: