HCMV gB is an essential viral protein which is involved in the early events of infection. It has been shown to bind to a variety of cell surface molecules such as heparan sulfate proteoglycans, integrin heterodimers and platelet-derived growth factor-a receptor [25] [26] [27] . In addition, HCMV gB has been shown to mediate fusion of viral and cellular membranes [28] [29] [30] . The protein is essential for viral entry and cell-to-cell spread but not for virion attachment, assembly or egress [31] . Three antigenic domains (AD) have been described previously. AD-1 consists of approximately 80aa between positions 560 and 640 of gB of HCMV strain AD169 [32] . It is the immunodominant region of gB since nearly all sera from HCMV-infected individuals recognize AD-1 [33] . Antibodies that bind to AD-1 can have virus neutralizing capacity as indicated by the fact that a number of AD-1-specific human mabs have been isolated which show various degrees of neutralizing activity [34, 35] . Polyclonal AD-1-specific antibodies, purified from human serum, are incapable of completely neutralizing HCMV even at high concentration, indicating that AD-1 is bound by antibodies with widely differing neutralizing activity [36] . It has been suggested that the competitive binding of neutralizing and non-neutralizing antibodies to AD-1 may represent a mechanism to evade efficient neutralization of cell free virus [36] .