Competitive binding of neutralizing and non-neutralizing human mabs has been described for gB [35] . We tested whether a similar phenomenon occurs for Dom I-or Dom II-specific neutralizing antibodies. A total of 14 non-neutralizing mabs, directed against different antigenic regions of gB was analyzed in neutralization assays for competition with Dom I-or Dom IIspecific mabs (SM10, 1G2, SM5-1). In no case did we observe a reduction in neutralizing capacity by addition of non-neutralizing mabs. The result of a representative analysis is shown in Fig. 6A . Although these data indicated that reduction of neutralizing activity by competing non-neutralizing mabs is not common for Dom I-and Dom II-specific antibodies, it is difficult to assess the relevance of this finding for the situation in human sera. For AD-1specific antibodies it has clearly been shown that the sum of antibodies, as present in human sera, is not capable of completely neutralizing infectious virus, indicating that non-neutralizing antibodies can constitute a significant fraction of the entire AD-1-specific IgG fraction [36] . To obtain more information on the neutralizing capacity of Dom II-specific antibodies in human sera, we purified Dom II as a GST fusion protein following expression in E. coli (Fig. S3A) . The bacterially derived peptide retained the capacity to bind all Group B mabs and the affinity of mab SM5-1 was similar for gB and Dom II (Fig. S3B ). Circular dichroism spectra indicated a homogeneous three-dimensional structure (Fig.  S3C ). Using this protein, polyclonal Dom II-specific antibodies were affinity-purified from pooled human sera (Fig. S4) . The affinity-purified IgG preparation was then tested in neutralization assays. 50% neutralization of input viral infectivity was achieved with an IgG concentration of approximately 0.2 mg/ml which is within the same concentration range as the Dom II-specific mabs (Fig. 6B) . In summary, these data provided evidence that gB Dom II not only represents an immunogenic domain on gB, but also that antibodies binding to it in general have potent neutralizing capacity. Similar experiments using Dom I could not be carried out since the Dom I domain does not fold correctly after prokaryotic expression and thus antigen for affinity purification of antibodies could not be generated.