The plasmids for envelope glycoprotein expression and the purified proteins used in this study were obtained from various sources; RSC3 core wild-type (WT) and its mutant Δ371I/P363N expression plasmids and proteins (kindly provided by Dr. John R. Mascola, NIAID, VRC, USA), HXBc2 WT and its D368R mutant gp120 plasmids (kindly provided by Dr. Joseph Sodroski, Dana Farber Cancer Institute, USA). The anti-HIV-1 bnAbs 2G12, VRC01 were obtained from the NIH AIDS Reagent Program (NIH ARP). Antibody 1418, specific for the capsid of human parvovirus B19, was kindly provided by Dr. Susan Zolla Pazner, NYU, USA. The plasmids for CAP256 and its N332A mutant were kindly provided by Dr. Lynn Morris, National Institute for Communicable Diseases, South Africa. The pAK100 and pAK400 plasmids were kindly gifted by Dr. Andreas Pluckthun, Department of Biochemistry, University of Zurich, Switzerland.