To measure the breadth of coverage and resolution offered by the panel of primers described in Methods (details in Table S1 ), we tested 92 well-characterized influenza virus isolates collected from human, avian, and animal species. Despite the extensive genetic diversity of this sample set, the broad-range primers generated amplicons from all isolates, and base composition signatures distinguished the isolates (Figure 1 ). Most isolates showed base compositions consistent with expected signatures for the corresponding H/N sub-types based on bioinformatic analysis of existing sequence data. Two of the isolates, however, showed previously unknown base compositions at several primer loci suggesting these might be novel influenza virus types; these are noted as ''Unknown'' in Figure 1 .