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:However, several lignin-degrading white-rot fungi seem to operate ligninolysis without expressing detectable
ligninase activity (Sannia et al. 1991; Garzillo et al. 1992; Srebotnik et al. 1994; Eggert et al. 1996b, 1997).
Category: