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 importance of C/EBP proteins initially has been demonstrated in tissue culture model systems of adipogenesis and hematopoiesis (Lin and Lane 1992; Freytag et al. 1994; Hu et al. 1995; Müller et al. 1995; Nerlov et al. 1998) and has now been firmly established through analysis of the respective knockout mice (Screpanti et al.
Category: