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:HLA class II expression can be induced on cultured thyroid cells by the addition of IFN-gamma, but not by adding IFN-alpha or IL-2 without IFN-gamma (Todd et al., 1985).
Category: