In order to obtain further information on potential structural domains of gB in the region between aa 100-448, we generated a three dimensional model of the trimeric conformation of the ectodomain of HCMV gB strain AD169 based on the crystal structure of HSV-1 gB [46] (Fig. 3) . According to the model, both the overall structure of the HCMV gB monomer and the organization of the subunits in the trimer were highly similar to that of HSV-1 gB, as expected from the degree of sequence similarity of gB between human herpesviruses (28% identity and 40% similarity between HSV-1 gB and HCMV strain AD169 gB). The individual domains I to V, which were previously defined based on the HSV-1 gB structure, can be clearly identified from the homology model of HCMV gB. With respect to potential antibody binding structures, the protein domains (Dom) I and II were of particular interest since they are located within aa 100-448 of gB. Dom I (Ile 133 to Thr 343 ) constitutes part of the trimer interface and is located proximal to the membrane, potentially containing the fusion domain [47] . The discontinuous Dom II is composed of two separate segments comprising residues Leu 121 to Asn 132 and Cys 344 to Ser 438 (Fig. 3) . Either domain contains a single disulfide bond which helps to stabilize the conformation of the respective protein domain [48] .