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:In order to determine the coding potential of each MuLVERV isolate, the ORFs for gag, pol and env polypeptides were compared to the MuLV references (Supplementary Table S2) (Shinnick et al., 1981; Urisman et al., 2006).
Category: