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:…first discovered, were thought to function as appetite promoters (Sakurai et al., 1998), and it was then found that their impaired system initiates narcoleptic symptoms (Chemelli et al., 1999; Lin et al., 1999; Mieda and Yanagisawa, 2002; Sutcliffe and de Lecea, 2002; Sakurai, 2005; Nishino, 2007).
Category: