We believe that further characterization of the gp41 binding antibodies is necessary; we performed additional ELISA experiments using purified gp41 protein and sera from our vaccinated mice. We were able to observe a slight binding of whole gp41 protein to the sera of mice vaccinated with Ad vector presenting MPER within the hexon capsid (data not shown). We speculate that this binding would have been increased if we were able to obtain an identical purified gp41 protein as that presented with our capsid-incorporated MPER sequence. This was not possible due to the divergency of HIV protein structures and reagents available. We also performed an ELISA assay with HIV pseudo particles and sera from our vaccinated mice (data not shown). The results were similar to that seen with purified gp41 protein. We speculate that it is difficult to detect binding of gp41 to the HIV particle due to the need of further optimization of this assay and/ or the limited amount of gp41 molecules available on a single HIV virion (,10-30 Env (gp120 and gp41) molecules per virion).