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 APP response has been observed in common pig infections (Grau-Roma et al., 2009; Pomorska-Mól et al., 2011a, b) but it is not known whether infection with H1N2 swine influenza virus (SIV) also induces an APP response.
Category: