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 viral VP4, VP6, VP7 and NSP4 genes were amplified with primer sets Con3/Con2, JRG7/JRG8 or GEN-VP6F/GEN-VP6R, Beg9-End9 and JRG30/JRG31, respectively (Gouvea et al., 1990; Gentsch et al., 1992; Matthijnssens et al., 2006; Esona et al.; 2009; Mijatovic-Rustempasic et al., 2011), and subjected to direct sequencing using the same PCR primers.
Category: