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:The action on CNS development of some of the factors contained within E-CSF has been analysed, including the role of the proteoglycans, FGF2, RBP/all-trans retinol system, and the apolipoprotein-carried lipids (Alonso et al., 1999,1998; Martin et al., 2006; Parada et al., 2008, in press).
Category: