The number of family-level primers for each family, and the number of genomes available for generating those primer sets, is given in Table 1 , for three alternative settings: short primers of 10-15 nt with T m 40-45 C, standard length primers of 17-21 nt and T m 55-60 C, and the same but requiring that each primer subsequence of at least 17 nt be unique to the target viral family relative to other viral families. At least one amplicon of 200-800 bp was required from every genome. Hypothetically, the worst-case scenario to amplify a target set of N sequences would require 2N primers. MPP requires on average only 37% or 45% of this number, for primers of length 10-15 nt or 17-21 nt without the requirement for family specific primers, respectively (averaged across families, omitting those families for which the computations were not run to completion). The most diverse families, in particular Bunyaviridae, Geminiviridae, Polydnaviridae, Reoviridae, Retroviridae, Siphoviridae and Orthomyxoviridae require so many primers that actually applying family-level amplification is probably infeasible. For these, more restricted target sets may be necessary, such as limiting to a single segment for the segmented families or to subclades, and possibly the incorporation of primers with degenerate or inosine bases. Some families with many genomes can be amplified with relatively few primers, such as Coronaviridae, Hepadnaviridae, Poxviridae, Togaviridae, Microviridae and Polyomaviridae. Using primers of length 10-15-mers or 17-21-mers, 66% or 63%, respectively, of the viral families have primer sets of 40 or fewer primers (20 primer pairs), which is feasible for typical multiplexes.